You talk as if the players play in full health all the time and "here comes the injury" after which they are not sure whether they are injured or not. The reality is completely different: these athletes have been through so many physical issues, that they know when something is completely off and very often they know what exactly is wrong with them, since they have entered the match suffering from it.
Let's say that they don't: they are concerned about it being serious: there are two scenarios then:
1) it is serious - in that case they have the help and can see if they can continue - in that case they forfeit their next service game as an exchange for getting help
2) it isn't serious - in that case they are penalised for not making the right call and "accidentally" disrupting the flow of the events. Small punishment for taking away from their opponent and the public
In either scenario there is no issue with proceeding with the game penalty.