Garnacho came off the bench to score United's final goal in the 3-0 win.
Ten Hag said, “First 20 minutes was hard, tough, but that is often (the case) in games, you have to get a foothold in the game.
“Once Andre (Onana) stopped the penalty, straight after, we scored a goal with Matthijs de Ligt, the game was all ours and we dominate the game from that point.
“I think we should have scored earlier on, we had the chances and we had the control, and we create some good chances to score the third goal earlier on.
“Of course, we are happy when you bring a sub on who has an impact and he (Garnacho) was the one who downsized the opponent to 10 with a good action and then a good finish.
“I think also the rest (of Garnacho’s performance), I was happy with this sub and that is how players should act from the bench and we need more from this.”