Animation distinguish between lower and upper body injuries. There was a way to know the kind of injury in FM10, I have not used it since FM11 so I can't guarantee it still works: go to the player profile after the physios have finished looking at him and you'll see what injury he's got. I think I stopped because the first time I did in FM11 I didn't see the injury; but I suspect it was one of those times when there's no recovery time, not because it stopped working like that.
To have an idea of how bad a knock can be, there's two things you can use:
- Player's body language/assistant's advice. If it says hindered by injury, wants to stay but something or other in the body language or the assistant is saying he should be subbed, then it's an injury that will become very serious if you keep him, it may be a long one even subbing immediately. If body language and assistant say he feels he can shake it off, then it's not a big deal and you can keep him. There's also in body language "recovering from a knock" that I've seen twice and seems to be akin to "feels he can shake the knock off".
- Player's fitness: the more it drops the more serious the injury is. I follow the rule that a drop higher than 20% is immediate sub regardless of the previous point, if it's below 10% I keep him unless the previous says sub, between 10% and 20% it depends on whether or not I feel I want to keep the sub for some tactical effect, how many subs are left, the importance of the player, the time until the end of the match or half and mood.
Anyway, you have to keep in mind that with any level of injury, the more time he remains on the field the worse it will be.