Judging by his latest tweet, it's angling toward being kayfabe. I'll give the WWE credit for doing it in a more creative way than has been done in the past. Usually, an injury angle is run entirely on TV, so everyone knows it's kayfabe from the beginning. Starting it online with tweets and website articles has people questioning, which is a good move. Cena's insistence that he'll be on Monday Night and the way the whole story has been broken leads me to think that it's probably adding some spice to the Ryback match. Then again, if he is really injured, they wouldn't just not having him on TV anymore. They would probably bring him out to Raw and have someone, Shield or Ryback, annihilate him to write him off TV. Although, on the flip side of that, when he tore his pectoral muscle in 2008 they didn't write him off. On the flip flip side of that, he was actually not the prime focus of WWE television at that time, believe it or not, so they may have found it unnecessary.
It's very hard to predict what's going on here, because of the entirely uncertain nature of whether the injury is real or not. My gut is telling me it's fake, but it's too hard to tell right now. While I've long been of the opinion a little time off for Cena would help everyone out, an Achilles tendon is about a year rehab minimum, and it can have long term consequences that never go away. Also, happening at a time when Punk is gone as well make for major problems. If he's legitimately about to be out for a year, WWE TV will need Punk back ASAP. In the meantime, asking Lesnar to work more dates on his contract now will become a necessity, and they may have to turn him face on the quick (something they should be doing anyway). Ryback may or may not be able to remain a main event player, since he was almost certainly going to be guided in the ring by Cena, and all of his heat (or pop) right now is really coming from the fact that it's Cena he's up against, so sub in Lesnar, or Orton, or Sheamus, and we begin to have a problem. A Cena-less and Punk-less WWE is a very talent bereft WWE, so hopefully this is indeed merely kayfabe.