In 4-3-3 you don't have typical wingers, you have inside forwards or depending on their skills you can put them as attacking mids with playmaking role. That would be Kaka's job and that in my opinion was supposed to be Sneijder's job at Camp Nou (1-0).
4 defenders
2 center mids with more of a defending role in the assigned area of midfield (Motta-Cambiasso in Inter, Alonso-used to be Lass/Gago in Pellegrini's and it could stay like this in Mourinho's but this time with Khedira)
1 center mid with a box-to-box role, responsible for bringing the ball forward and playmaking (most of the time Granero in Pellegrini's team and Sneijder while playing 4-3-3 with 2 inside forwards in Inter, that's the kind of player we need because Granero doesn't fit to this postition, Schweiny would do the job really or maybe if De La Red returns...), also we could assign this player to a flank to be more offensive if we need him to just like we did with Guti all along when he played first team eg. against Deportivo, not a proper center mid, but playing slightly to the left, that's where Di Maria plays for Argentina (where he's wasted quite frankly)
1 attacking mid with much more of a playmaking and carrying the ball on the opposing side of the pitch role (Kaka in ours, Sneijder in Inter) or an inside forward (that's the reason why IMO Di Maria doesn't really fit, I think he's more of a casual winger than a Ronaldo kind of guy)
1 inside forward just behind the striker, usually free to switch flanks with the other guy who he's in line with (Ronaldo/Pedro Leon even though he's more of a winger but if he will be Ronaldo's backup then he will get used to it, Eto'o)
1 proper striker (Higuain, Benzema but I don't really know about him as he seems to be more of a centre forward than a striker as he gets caught offside REALLY often and likes to work down the flanks helping out the teammates, Milito in Inter)
Don't get me wrong. I mean, we have overload of players and we can't fit em all and quite realistically, we won't bench Kaka (unless he undergoes surgery which will remove the problem) so that would be the only role we would see him play. We won't fit 4-2-3-1 because we won't have a Xavi kind of person in the middle and it won't work without the guy.