I read that it's kayfabe because he's filming another movie soon, but I'd also believe it to be a real injury given the unconventional way it happened and how it was displayed on TV, meaning it wasn't. Usually WWE's all up in our face when they're trying to sell a kayfabe injury but if this one was planned, then they kept it surprisingly subtle, even going as far as to bring it up a week after it all supposedly happened.
It is a bit strange that he was able to pick up 264lbs Drew McIntyre without any issues, to then start limping after he hit the move. Then again, Cena's a monster when it comes to pain and working through it, so maybe he just put up a front as long as he could