Perhaps John O'Shea might make an appearance at Centre Back. This would mean that perhaps Rafael, Fabio or Wes Brown might have a run at Right Back, although this is highly unlikely, given that Gary Neville is playing more and more frequently than he was earlier in the season.
Will Fergie see this as an opportunity to bring a fresh face or a two up for a brief stint in the senior squad, just to get a taste of the "big time" football? Could it be Ritchie De Laet's chance to shine? Or maybe Corry Evans? I would not be surprised if we see one of these players appear at Centre Back for one, maybe even two or three games, but to be honest i feel that none of these players will start frequently during the time Rio is out of action.
Who knows, maybe Fergie will even temporarily change to the 3-5-2. This is most definately a more outlandish prediction, but it could be a chance for him to try new things. The only problem would be finding 3 players who can play adequately in a 3-5-2 formation, as it is fair to assume that neither Evra nor Neville, or any other designated Wing Back, would fit into a central role of a 3-5-2.
Personally, I feel that the burden will be shared by Jonny Evans and John O'Shea, with Wes Brown taking on what is usually O'Shea's role of a back-up all-round defender. It will be interesting to see how Fergie responds to the situation.