Martin O’Neill believes Celtic are now counting the cost of not replacing Arne Engels despite knowing since January that he was set to move on.The Belgian was the subject of substantial bids from Nottingham Forest at the turn of the year.While Celtic resisted selling the midfielder in that window, they agreed to let him go when West Ham tabled £22million in the summer without having a replacement lined up.Engels’ absence has been sorely felt since with Celtic crashing out of the Champions League to LASK, losing heavily to Rangers in the Premier Sports Cup and going down to Ferencvaros in the Europa League on Thursday.The Parkhead club tried and failed to sign Torino’s Lithuanian midfielder Gvidas Gineitis in the closing days of the transfer window, a matter – O’Neill now admits – has left him short in a key area. O’Neill admits Celtic’s midfield lacks running power after they failed to replace Engels (left)‘I think that in an ideal world, it would have been greater to have the likes of Arne Engels and that type of running power,’ he said.‘And that’s really what we were trying to do when we knew that Arne was going to leave. So, it didn’t materialise, but that’s something that we would certainly have to look at.’Arne, I always felt he was going to be leaving anyway. Trying to get a like for like, we looked at a number of players who could do that.’For one reason or another, it didn’t materialise. So yeah, he’s been a loss in terms of that running power he can give you.’Ahead of the first league meeting of the season with Rangers at Parkhead at lunchtime today, O’Neill feels the acquisition of a replacement for Engels must be a top priority as soon as the January transfer window opens.’You’d have to. Very much so,’ he added. ‘January seems a million miles away, but it comes round relatively quickly. You’d have to do something like that there.’Between loans and permanent signings, O’Neill welcomed 10 new faces to the club over the summer.After a decent start, the club has lost three significant matches with the manager still looking for the best formation to suit the players at his disposal.Asked if the summer recruitment drive began with a particular formation in mind, O’Neill said ‘Not necessarily. If you’re going to play whatever system you want, you need strong running players in your side. You definitely need that there. You need a bit of strength.‘That was something that I thought was lacking from last season. You’re trying to get the right person in to do that and sometimes you get frustrated in the transfer market if it doesn’t materialise.‘But that’s really something that didn’t necessarily mean fitting just one system, it just meant players being able to play whatever you do.‘Really, for all of us talking about this and thinking that we’re in this modern era, it is about players being able to break systems down.‘I watched Manchester United (against Brighton) the other night, who were 2-0 up and coasting, and allowing a team to come back into the game because they make mistakes at the back that you wouldn’t be expecting Premier League teams to do.’