Maguire was out of favour with his club side, making just four appearances in the Premier League before going to the World Cup with England.
The 29-year-old was an ever present for the Three Lions in Qatar and put up some impressive performances as Gareth Southgate's side reached the quarterfinals before bowing out to defending champions France.
Maguire will be back to Old Trafford to compete with Raphael Varane, Lisandro Martinez and Victor Lindelof for a starting berth in defence and Ten Hag expects his captain to slot in seamlessly when given his chance.
"I can only back him. I've been asked many times if he is good enough [to play for me] and it is clear he is good enough to play at the highest level," the United manager told reporters.
"And then it is up to him to show that confidence on the pitch and he didn't show that in all the games [for United].
"When he is playing with his confidence like now, he is a massively important player for us and that is what everyone expects. That's what Harry expects from himself.
"He has that high standard, he wants to play at the highest level, he wants to lead the defensive line, so it is up to him. I'm sure he has the potential to play in our style and fit into our system.
"He has, for England, almost all the time good games. He had a period in Manchester where he performed badly and then, of course, there are difficulties but when he is confident in himself, he will put that on the pitch.
"When he is working hard you will get that confidence and you are seeing that with the way he is playing for England.
"We want him to bring that back to Manchester with him so he can bring it on the pitch for Manchester United. I expect from him, the team knows what they expect from him. If he does that he will be a great player for us."
