Arsenal FC have had the much better of the possession in the first half of their FA Cup tie against Manchester United, but it always seemed like the two teams were about equally likely to score. Fabio da Silva's goal in the 28th minute exemplified that, as it came at the end of a great counter-attacking move. The 1-0 scoreline, along with the way United are playing at the moment, is probably going to force both teams into some changes in the second half if the score stays the way it is at the moment.
Tomas Rosicky was somewhat of a surprise exclusion, and he will probably make an appearance at some point. Denilson hasn't been terribly effective in the midfield for Arsenal, and he will probably be the man removed. If things get really desperate, Wenger could also opt to remove either Denilson or Abou Diaby for Marouane Chamakh.
On the other side, there's no way that Fabio and his brother Rafael can keep up the pace that they have been playing at for 90 minutes. Both of them have been absolutely everywhere, both going forward and tracking back, and they both appear to be exerting all of the energy that they have. It would be surprising if either of them was able to play the entire 90 minutes for United.