Its always a little embarassing when a sub is subbed. Probably why he was upset. If he had started, maybe it wouldnt have affected him as much.It was ok Poch subing him even though I'd have made a different change. I think though Son is the kind of player who in those circumstances will take it personally and feel ashamed by it. You could see he was upset. So I hope Poch gives him some reassurance.
Edit: just saw someone else mention something similar..