Arsenal will be without half of their first-choice back four for the start of the Premier League season, Mikel Arteta has confirmed in a disappointing injury update.The Gunners knew they would be going into the new campaign without centre back William Saliba, who sustained a back injury while on World Cup duty with France.The club had already confirmed that his issue would require ‘a period of rehabilitation’, splitting last season’s star central duo of Saliba and Gabriel.But there had been hope that Timber, who missed the World Cup with the Netherlands, would be fit, but it appears he is still some way away from returning to action.’I cannot be very precise with that,’ Arteta said after Arsenal’s pre-season defeat by Borussia Dortmund on Sunday. ‘Jurrien is progressing really well, he’s on the pitch, he’s doing quite a lot right now, but is still weeks away. I cannot say how much.’William is in rest mode at the moment. For two weeks he had to do almost nothing, don’t aggravate the injury. It’s going to take some time to heal, we know that. Arsenal are set for a period of time without William Saliba (left) and Jurrien Timber (right)’As quick as we have more news we can inform.’Ben White and Cristhian Mosquera are currently set to deputise for the duo, but Arsenal could go into the market to buy replacements too.Saliba was captured saying ‘my back is gone, my back is gone’ after being withdrawn in the second-half of Les Bleus’ defeat by Spain at the World Cup, with reports in his home country suggesting that he could be sidelined for an extended period of time.Arsenal and Saliba are believed to have opted against surgery to heal the issue.Timber, meanwhile, played briefly at the end of last season but missed the World Cup with a persistent groin issue. Arteta said: ‘We have experienced that in the past, unfortunately. When he’s been available, his stats are incredible.’We know that we have lost an incredible player. Same with Jurrien.’He added: ‘We need to improve with the players that we have.’