We have two wingers who can directly cover for Nani. It's not as easy to find your replacement for Vidic and Rooney, in those positions the difference for the cover has more of an impact. Not to mention that it's possible the risk for Nani making that existing injury worse is higher than that of Vidic or Rooney, which again would be the recovery argument and how not all players are the same. Maybe they are able to play through it, where he can't. My point is when fit, Nani is always going to start over Park or Young, and Fergie would have put him in the crunch games had he not been recently injured, or recovering from an injury at the time. It's strange how whenever Nani is dropped, it's always following a recent injury, weird huh?