Fair enough - there are like-for-like replacements to whom this argument wouldn't apply, though.Surely it has something to do with tactics. Teams study and practice all week for the opposition so will start doing that with the fittest players for each position in the hope that, barring injuries, it can be tweaked later in the game depending on how it is going.
You wouldn't start with someone who might only manage 20 minutes and then have to revise your whole game strategy.
The 20 minutes timescale was just an off the top of my head, worst-case scenario - that's the point I was clumsily trying to make. You might just find that the recovering player is good for 45, 60 or even the whole match. In that case, afterwards you'd be much better informed than just having seen him trot around for a few minutes at the end of the game.
Just different opinions, that's all