Pretty much this. Had a quick read up, could just be a small tear and not require surgery, would still swell up if that's the case. Fingers crossed it just swelled a little bit and it's precautionary, from the quotes ('Santon's knee looked a little bit unhealthy after training), it sounded like he completed the training session, and then it swelled up after. Would hazard a guess that a larger tear would stop him from continuing? Perhaps it wasn't that bad #overlyoptimistic
I'm hoping this is the case.