Achilles injuries are pretty serious. Wasn't Ryback out for a year because of an ankle injury?
Not to sound callous but things like this can turn out to be a good thing. Just imagine if Cena for out for at least a few months and try had to push someone new (gasp) in his absence. A few different ways this could turn out:
-Ryback actually wins the title
-Cena fights through the injury and retains, but because of a number of things - the match being so brutal, Ryback attacking him afterwards and/or The Shield attacking him afterwards - he has to forfeit the title
-Cena retains the title but then the next night, Mark Henry, who thinks he deserves a title shot (don't forget the night after Wrestlemania), attacks Cena and does the old "put the chair on the ankle and then stomp on it" spot and cripples Cena, putting him into the Hall Of Pain. While Cena howls in pain, Henry screams "That's what I do!" And the title is vacated.
If Cena does miss time, then him vacating the title is the best option, in my opinion. When's the last time we had a huge tournament to declare the undisputed world champion? The "Deadly Games" tournament from Survivor Series 1998? They could hold a huge tourney with almost everyone in the WWE that matters - Ryback, Henry, one of the members of The Shield (if they win the tag titles, then one of them could decide to go after the world title as well), Orton, Show, Sheamus, Kane, Bryan, Del Rio, Kofi, Caesro, Barrett, Jericho, Swagger, etc.
Even Ziggler could enter it, saying he's gonna now own both world championships and unify them. Remember when he stated that at the Royal Rumble earlier this year?