Sancho did not make the bench amid talk of a swap deal with PSG for Manuel Ugarte.
Ten Hag said afterwards: "First of all we need squad depth and we need it this season to be survival of the fittest. I can only put 20 players in the squad, I have to make choices and in the week, he had an ear infection but he was not 100 per cent fit.
"He could play but if we had the choices, we preferred to put other players on the bench but that will change in the season as we have to rotate, because we can't play so many games the same. You see how important the subs are and you see it is about balance. Sometimes it is frustrating about the team, the club and there will be more players in this position.
"We expect them to fight for the club, as we have so many games to play for the club. Everyone will get their minutes and then they have to prove it."
On again playing Alejandro Garnacho from the bench, Ten Hag also said: "I explained the same about Zirkzee, it is the same about Alejandro, you saw his background, he played in an FA Cup final and then in Copa America. He had one game and then he had his break. He needed a break and we had to bring him into the tour.
"Fitness-wise, it would have been the best decision to be in the tour, to work on his fitness. After a long season as a young player, he needed a break. He came late into pre-season and we have players in wide areas who have performed very well. That is why he started on the bench. It is the same story about Zirkzee, Sancho, Garnacho too. He will have his minutes and he will have starts. That will come in the coming weeks, months."