I can see several potential problems with putting out a balanced team up to our usual level.
One is that both Yaya & Barry are the two main players in our whole squad who go & get the ball deep. If Utd are smart, they could press us there & totally fuck our passing game up. To counter that, we need to get Silva/Nasri/AJ whoever dropping in there to give Nige options, or Milner to play a blinder.
Two: They score a hell of a lot of of goals from set pieces & we may be lacking several of our better aerial players. I hope dzeko or Mario play,or we could have a problem.
Three: Milner has not played many games for us in that Gareth Barry position & Nige hasn't been very good so far this season. Neither read the game like Barry does.
As mentioned by Joey above, here's a case for playing Zabba alongside Nige & letting Milner bomb on ala Yaya Toure. Failing that, Milner's performance in all areas may determine the outcome (assuming we don't just run straight through the fuckers). He needs to pass well in particular.
It's easy to see why Bob wants DeRossi; passing from the back, filling holes when we lose posession & strong in the air. Does all the stuff we need for this game & others to come. It's also easy to see why Nige (great as he is) may become obsolete next season.