If WWE is smart they will have Cena not wrestle till Extreme Rules (unfortunately that means we will get a whole lot of Cena talking up till then) , then they have him wrestle at Extreme Rules (pray that his Achilles does not give out) and have him drop the title to Ryback and have Ryback put him out of action so to speak.
Then rest Cena much like how Punk is getting Rest now to prevent further (maybe even career ending) injury. I know WWE would feel they are pretty screwed if they both without Cena and Punk for a couple months, and I will admit that is not ideal.
However they have plenty of guys they could push toward the WWE Championship to feud with Ryback (depending on him being Face or Heel post Extreme Rules), Daniel Bryan, Sheamus, Randy Orton, Mark Henry, Big Show, Swagger, Del Rio just to name a few.
Heck they could some even more interesting things and have Dolph Ziggler feud to become a Unified Champion (unlikely they would do this though sadly IMO) OR have Triple H feud with Ryback and have a couple months feud where Triple H wins the title from Ryback for a bit. Then either Triple H loses it back to Ryback before Cena returns to continue his feud with Ryback OR what I hope they do is have CM Punk comeback and Feud with then WWE Champion Triple H in what could be an epic feud for the title!