A large part of this would depend on how close to WrestleMania Cena gets hurt. Obviously, if it's the week before, they wouldn't have time to do anything...but if it's, say at the Royal Rumble or the Elimination Chamber?
Rock would still fight somebody. They could simply go with the closet thing to Cena today, which would be Randy Orton or CM Punk...or they could go the 'up and coming' route, somebody like Miz or Barrett. I don't really see the point of Triple H or Austin, honestly.
Or they could go HUGE. There's three options for opponents that could match the hype and anticipation of Rock vs. Cena.
Have The Rock come out and talk about all he's done in the wrestling business, everybody he's beat, all the titles he's won, blah blah blah. But there's one thing he hasn't done...
A."I've never beat The Undertaker at WrestleMania." Rock vs. Undertaker sells itself. This depends on who they have lined up for Undertaker...if it's Brock Lesnar, I don't think they toss that away, but if it's Kane or Triple H, I think they'd plug The Rock in instead.
B."I've never wrestled Shawn Michaels." I know HBK said he's done, but desparate times call for desparate measures, and I think if anything could make him come out of retirement for one more match, this would be it.
C."I've never beat Brock Lesnar." Lesnar ended Rock's last WWE title reign and broke his record of youngest ever WWE champion. Again, if Lesnar is scheduled to fight Undertaker, this option goes out the window. But if he's available, I think they'd throw a ton of money at him to come back for this match.
Rock/Cena would still happen at some point...when might depend on how severely Cena is hurt. Would they do it at SummerSlam? Or save it for WrestleMania 29?
My guess is Rock would fight CM Punk at WrestleMania 28 and John Cena at WrestleMania 29.