Maguire, ruled out of United's derby at Manchester City on Sunday due to injury, gave away a penalty against Germany on Monday and lost possession before the visitors' second goal in the 3-3 draw.
His latest sub-par performance led to renewed talk about whether the 29-year-old defender, who has been out of form and favour at United, should still be in the England team, just weeks away from the World Cup.
But Ten Hag, like England boss Gareth Southgate, retains faith in Maguire and is certain he will improve.
"First of all, of course I have to coach him, I have to back him," Ten Hag said at his pre-match press conference on Friday.
"But I back him because I believe in him. In the period I worked with him in pre-season he was good, I think I would say really good -- training and games.
"Then he fell out (of the starting line-up) but it's also to do with the good performance of the centre-backs who are playing now."
Argentina's Lisandro Martinez and France international Raphael Varane have been Ten Hag's preferred centre-back partnership in recent games.
Ten Hag said everyone in United's squad and on their coaching staff still believe in the former Leicester centre-back, who picked up a knock on England duty.
"Now it is about him," he said. "That is what I told him. I'm sure he can do it. He will turn around this. I am really convinced of that."
United's match on Sunday will be their first Premier League outing since they beat Arsenal 3-1 on September 4 -- the Crystal Palace and Leeds fixtures were called off following the death of Queen Elizabeth II.
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