Loss of condition after minor injury

I'm puzzled as to why, when a player receives a very minor knock (not a 'green cross' or anything), the sort of thing which one sees quite often in a match, where the trainer comes on, gives him a quick burst of 'magic spray' and he's fine again, there is a condition reduction of around 20%, which lasts for ages. This figure seems too high to me. What do people think?