It seemed real. If it's a work then the way that the segment went down, with the lack of Bray hitting his finish, was really convincing.
I fear that it looked like a potential ACL tear, what with Cena claiming he heard a 'pop' and that picture doing the rounds which appears to show heavy swelling, pretty soon after the incident. If this is a case the good news is that he could probably make it through Wrestlemania, protecting himself as much as possible through to the event and then getting it fixed after Wrestlemania, but he'd be looking at a long spell on the sidelines from there.
If he can't make Wrestlemania, the obvious solution would appear to be holding off on the Shield split and running another Shield/Wyatts match in April. It would also pose the theory that Vince might go the extra mile to try and get CM Punk on the card, if he felt that it lacked star power without Cena. I have to say I'd be devastated if Cena had to sit it out, with 30 being my first live Wrestlemania.