Fact is Arsenal were behind United a long time ago, then you had a whole host of injuries. The chances of you winning the league have been getting slimmer and slimmer since end of January time. You don't have an easy run in, harder than the teams around you, and rather than looking at United and hoping for a few dropped points I would be looking at Chelsea behind you, and trying to secure second place and the auto quali that goes with it.
I don't think there is anything wrong with the Arsenal team, but you do rely heavily on key players, Fab can be replaced with Nasri , but you have no striker to replace RVP, I think if one player is needed its a good second striker. I don't think you need Villa, but a consistent striker capable of filling the void if RVP gets injured. I though Chamakh could do it but he seems to have lost form.