The former Ajax coach recently surprised many when he retained the services of Maguire as team captain despite his high-profile errors.
The 29-year-old struggled for form in Premier League last season and was part of a Man United squad that narrowly qualified for the Europa League.
But despite giving him the vote of confidence, Ten Hag has told Maguire he must prove that he deserves to be in the starting XI at Old Trafford.
“Harry is really impressive and I expect a lot from him,” Ten Hag told reporters via Daily Mail.
“I always see the captaincy as an issue that I dictate.
“The team building for me is an important point and I always talk about a group of leaders. The captain is a really important one and I’m happy with him.”
However, Lisandro Martinez joins Man United this week for £55.3million and Raphael Varane and Victor Lindelof will be providing even more competition for places in central defence in the coming season.
“We have good centre-halves, and Harry is one of them,’ added Ten Hag. ‘He can play on the left and right sides because he is right-footed.
“But there is also internal competition, and that is what a club like Man United needs. You cannot win with 11 players. We need a squad, especially this season, with so many games.
“I think he is (a first-choice player). He’s proved it in the past, but he has also to prove it in the present and in the future. I will support him everywhere I can. In the end, he has to do it by himself, and he has the qualities to do it.”
Meanwhile, Ten Hag has dismissed fears that Martinez will struggle to adapt to the English game due to his size. The Argentina international stands at 5ft 9in, which is small for a centre-back, particularly one coming to English football for the first time.
But Ten Hag said: “I think he’s not the tallest, but he is quite good in the air. I feel comfortable with that. Of course, you need the right balance. He has good timing. That’s one of his capabilities.
“He also brings a South American aggressiveness, control and spirit to the team. I think it will fit really good with Manchester United and the way we play.
“I analysed United last year in the left part of the defence. He is left-footed, and that is an advantage in possession and defending. We are bringing him in to not strengthen the squad, but the team.”