I don't think it was incorrect to start with him and he could have scored within ten mins. Given Dembele's hip problems, it made sense to bring him on in the second half. Poch wasn't expecting our defence to gift two goals.It was all Son's fault.
If he hadn't played so well at the weekend then Poch wouldn't have bottled the correct decision to leave him on the bench for this game.
Poch playing Son,. forced Alli out of his position causing the knock on effect of messing up his, Dier's and Kane's game.
Son's substitution at half time points clearly to this.