Interesting thread, not all of which matches with my experiences!

I also don't get GK injuries very often (two in 33 seasons) and I don't have a GK on the bench. When they do happen the GK has continued to play and both times their performance has gone up after the injury. So I don't think they are automatically replaced by an outfield player, but my sample size is small!

This is just my assumption from watching matches, but I think there are two types of injuries, with the text commentary explaining the difference. There are serious injuries where the player is unable to continue (which is what it says in the text commentary), and less serious injuries where the player can continue. For the serious injuries the player goes off immediately and if you don't put on a substitute you will play with one less. For the less serious injuries I don't believe they go off automatically. I'll see if I can get some evidence of this next time I have an injury... or evidence that I'm wrong!