The fullback was impressive in the 1-0 opening night win.
Ten Hag said, "Subs are always important and it was good to see in the first game that you bring a sub on and he scores the winner.
"Everyone sees how important subs are and they have to be ready.
"Against Fulham it is always a difficult game, but we defended and pressed very well. After 10 minutes we found our spell in the game and we kept them under pressure. We created a lot of chances and we should have scored earlier and that is the only criticism of the team - kill in the box.
"Two season ago (we had) the most clean sheets in the Premier League. When we have a consistent back four we will defend properly. If you don't have a consistent back four the gaps will come.
"I know Noussair Mazraoui like this. He is a great defender, but he is very composed on the ball - he is not in full condition, when he gets 100% fitness he can do much more. He has to build his fitness."