Sancho was left out of the lineup for the second game in a row, as reports of interest from Juventus and PSG continue to swirl.
"I had to make a choice," said Ten Hag.
"We had some problems, we need a full team. He is one of those with problems and he has to compete for his position."
United were beaten 2-1 at Falmer stadium thanks to Joao Pedro's 94th minute winner.