The defender is not available to face Manchester City at the Etihad Stadium on Sunday because of injury, but Ten Hag said the 29-year-old can get back to his best.
"I have to back him but I back him because I believe in him," Ten Hag told a news conference on Friday.
"In the period I worked with him in preseason was good, really good, training and games, so then he fell out [of the team] but it's also to do with the centre-backs playing now and even after he wasn't in the team he trained really well and the quality was there.
"What you see is high potential. In the dressing room, the coaches and the players believe in him. That's what I told him. I'm sure he can do it and I'm convinced of that."
The Dutchman will have to come up with a plan to stop Erling Haaland, who has scored 14 goals in 10 games this season, but the United boss says he is not focused on the Norwegian striker.
"We don't play against Haaland, we play against Manchester City," Ten Hag added. "We are convinced of our capabilities and if we act as a team in and out of possession we can beat opponents."
The derby at the Etihad will see Ten Hag reunited with Pep Guardiola after the pair worked together at Bayern Munich.
Guardiola said last season that Ten Hag could possibly be a future manager at City, but the United boss said he made the right choice to take over at Old Trafford.
"It's a nice compliment, first, but I'm 100 percent convinced [I made the right decision]," Ten Hag said.
"I chose Man United with everything in it and I didn't regret it for one second until now."