As opportunity knocks for a number of Manchester United’s academy stars in pre-season, only the chosen few will be part of the first-team squad once the new Premier League campaign gets underway.United are open to the majority of the Under 21s who have featured in the opening friendlies against Wrexham and Rosenborg going out on loan to further their development, with the exception of players like Shea Lacey, Jack and Tyler Fletcher and rising star JJ Gabriel.For several youngsters who have been on the first-team fringes for some time now, the move could be permanent after the club made a decision on their futures this week.Goalkeeper Radek Vitek, left back Harry Amass and midfielders Dan Gore and Toby Collyer all spent last season out on loan and have been told they are unlikely to feature for United this season either.In the case of Vitek and Amass, United are prepared to sell both players or consider another loan with an option to buy. Until then, both players will remain part of the first-team squad and train at Carrington.Czech keeper Vitek, who has played more minutes than anyone against Wrexham and Rosenborg, is a target for Middlesbrough and a host of other Championship clubs after starring on loan at Bristol City last season. Michael Carrick has given a lot of game time to his young academy stars in pre-season A host of United youngsters got a runout in Norway, including Shea Lacey (front left), Harry Amass (centre) and 15-year-old JJ Gabriel (back right)Amass played in both games and got a goal and an assist against Rosenborg. He is valued at around £6million after spending last season on loan at Sheffield Wednesday and Norwich.With United still in the market for a third central midfielder after paying a combined £85m to sign Andrey Santos and Youri Tielemans, Gore and Collyer are believed to have been told that their best hope of regular football next season will be with the Under 21s.Hull City retain an interest in re-signing Collyer following his loan there last season, and Gore is available again after a successful loan spell at Rotherham.There is no shortage of other United youngsters looking to impress in the remaining four pre-season games against Atletico Madrid, Paris Saint-Germain, Leeds and AC Milan, even though opportunities may be harder to come by as United’s World Cup stars return to action.The club are famed for their youth system, and a feature of United’s first two pre-season games has been how well the academy players seized their opportunity in Helsinki and Trondheim.Lacey scored United’s opening goal over Rosenborg on Friday after he was named in the starting line-up, and Jaydan Kamason starred with two assists when Michael Carrick brought on the kids in the second half and they stretched the lead to 5-0. United’s first-team stars will be looking over their shoulder with so much talent waiting in the wingsIn the first game against Wrexham, the academy side played more fluent football than the seniors who went 1-0 down before half-time, but United couldn’t find the equaliser their performance deserved.Lacey will be looking to kick on after making his first-team breakthrough last season along with the Fletcher twins, and Gabriel is edging ever closer to a debut. Jim Thwaites is also understood to be particularly highly-rated by Carrick and his coaching staff even though he hasn’t played yet. Here Daily Mail Sport takes a look at the exciting youngsters knocking on the first-team door.SHEA LACEYThe only outfield player from the academy to make the starting line-up against Rosenborg, Lacey justified his inclusion with a well-taken goal just after the half-hour mark to set United on their way to a comprehensive victory.The 19-year-old from Liverpool is predominantly a right winger but demonstrated his versatility in Trondheim by playing in the No 10 role and drifting wide to link up play.Lacey’s star quality was underlined when Thomas Tuchel called him up to train with the England senior squad in October, and he made his first two appearances for United before being sent off on his Old Trafford debut against Brighton in the FA Cup.He re-appeared at Brighton in the Premier League on the final day of the season, though, and we will undoubtedly be seeing more of him this term. Lacey made four first-team appearances last season, including three in the Premier League Lacey saw red on his Old Trafford debut in the FA Cup against Brighton for throwing the ball away in a show of dissentJACK FLETCHERThe older of the Fletcher twins – who joined United from Manchester City for a joint fee of around £1.25m three years ago – Jack was named in the starting line-up for the Wrexham game and performed well in the No 10 role before Carrick made wholesale changes at half-time.He and Tyler play in central midfield, but they are expected to stay at United this season despite the club’s £85m investment in Andrey Santos and Youri Tielemans, and plans to sign a third midfielder.Jack is seen as a more creative player and tends to operate further forward, although the England youth international can also be used as a right winger or full back.The 19-year-old made three first-team appearances last season and will be aiming to add more this term. Jack Fletcher tends to operate further forward in midfield but can also be used as a right winger or full backTYLER FLETCHERHe didn’t start against Wrexham but at least had the distinction of captaining United’s younger XI in the second half in Helsinki, before coming on as a substitute along with his brother when Carrick made changes after an hour against Rosenborg.This could be a big season for Tyler after he made his senior international debut for Scotland in a friendly against Bolivia last month and then got a late call-up for the World Cup following an injury to Billy Gilmour.Like Lacey, he played for United at Brighton on the last day of the season in May, and those two are understood to be at the front of the queue for first-team opportunities this year. Tyler Fletcher had the distinction of captaining United’s younger XI in the second half in Helsinki against WrexhamJJ GABRIELThe most eagerly anticipated appearance in pre-season is likely to be made by the young man who doesn’t turn 16 until October.Gabriel was included in United’s squad for the first time for the trip to Norway last week, but was one of only three players from the 25-man party not to get on the pitch.It’s only a matter of time before the player dubbed Kid Messi gets his chance, after Premier League rules prevented United from naming him in their squad last season because he was only 14 when it began.Gabriel still made an impression, though, as he was called up to first-team training in the autumn and starred for the Under 18s with 29 goals in 32 appearances to win the Under 18s Premier League player of the season award. Expect to hear a lot more about this boy. JJ Gabriel has only just turned 15 but could be in line for a Premier League debut this seasonJAYDAN KAMASONThe England youth international was United’s standout performer in the second half against Rosenborg, getting forward down the right flank to set up goals for Harry Amass and Ethan Williams.Kamason struggled with injuries last season but is hoping to have more luck this term, and has certainly made a promising start in pre-season.The 19-year-old is prepared to go out on loan to further his development, but has his sights locked firmly on a career at Old Trafford.‘If I need a few loans or whatever, we’ll have to see, but I’m trying to get to that top level – and that’s Manchester United. You have to be ambitious,’ he said after the game in Trondheim. Jaydan Kamason struggled with injuries last season but is hoping to have more luck this term, and has certainly made a promising start in pre-seasonJIM THWAITESAlthough he has been left on the bench for both pre-season games so far, Thwaites is said to be particularly highly regarded among the current crop of Under 21s.A gifted midfielder and dead-ball specialist, the 18-year-old has seized his opportunity to train with the senior squad since Ruben Amorim first invited him in, and was rewarded by Carrick for his performance during the training camp in Dublin in April with a place on the bench at Chelsea.With United strengthening in midfield, they have to make a decision about whether to keep Thwaites or send him out on loan after a planned move in January was scuppered by injury. Jim Thwaites is a dead-ball specialist and is particularly highly regarded among the current crop of Under 21sJACOB DEVANEYDevaney has come off the bench to play in central defence in both pre-season games despite preferring a midfield role, and got on the scoresheet against Rosenborg with United’s third goal from a corner shortly after replacing Ayden Heaven.The 19-year-old excelled as a midfielder on loan at St Mirren during a difficult season for the Scottish club, and is ready to head out again if it boosts his hopes of playing for United. Jacob Devaney struck for United in their 5-0 pre-season friendly win over Rosenborg in Norway last week‘The main aim for me is to play for Manchester United week in, week out,’ said the Republic of Ireland Under 21s international in Trondheim. ‘Obviously, I want to play first-team football. So whether that is at United, learning from the best for the next year, or if I have to go on loan again and gain more valuable experience, that’s what I’ll do.’PS: Not all the young players knocking on the first-team door at United this season will have come through the club’s academy.There is great excitement around teenager winger Tynan Thompson who has moved to Old Trafford from Tottenham in a deal worth £4m plus £4m in add-ons.Thompson will be part of the senior group at Carrington, but the 18-year-old hasn’t trained with them yet and it’s too early to tell if he will feature in pre-season as United’s World Cup contingent continue to report back.