Nasri could step into Fabregas's boots, so could, Djourou, the problem is Nasri is worse, just less bad than Djourou would be as a replacement.
Our first 11 is entirely incapable of beating the top teams and therefore winning any titles right now, dropping the best player in the first 11 for someone else ALREADY IN that first 11, pretty much brings Nasri in from the right wing to the middle and pushes , under Wenger, Theo into a consistant starting spot.
Its not Fabregas to Nasri, its Fabregas, to Theo, and a simply ridiculous drop in quality, from a starting 11 who already are not good enough. There isn't a player in the league who can replace his assists, few who could score as many as he did this year, Nasri certainly won't, neither will Rosicky, or Arshavin, or Ramsey or Walcott.
As with Utd, you can easily lose one great player if you put someone half decent in his place and increase the quality of the rest of the squad to compensate. IE our current starting 11 including Fabregas wouldn't be as effective as a team with Buffon over Almunia, Diarra over Song, Ireland instead of Diaby, Eboue over Sagna, a proper CB and a real winger or two.
The problem is, as it has been for 4 years, is that Wenger thinks Diaby, Denilson, Song, Djourou, Theo are good enough. The massive problem is, managers don't replace players who they think are great, and why would they. The problem is his complete inability to see how useless Theo, Diaby, Denilson, Sagna are, and if he won't replace them with better players we can't improve and as he probably won't, Fabregas's loss will kill us completely.