The England international has fallen down the pecking order behind Lisandro Martinez, Raphael Varane and Victor Lindelof, with even left-back Luke Shaw having played in central defence, leaving Maguire with eight starts in the league this season.
Maguire became the world's most expensive defender when United signed him from Leicester City for £80 million ($100.97 million) in 2019. His contract is due to end in 2025.
Asked about Maguire's future, Ten Hag told the Times: "Let's say I'm happy he's here and when we needed him he did his job. But it's also a decision he has to make."
The Dutchman said Maguire was putting in 100% effort in training and had played an important role as club captain but acknowledged the player would not settle for a place on the bench going forward.
"No one would be happy with this situation. He is not as well," he added.
Ten Hag added that goalkeeper David de Gea will remain at the club next season but said the Spaniard, who kept the most clean sheets in the Premier League this season, would face competition for the gloves.
"I will not say he'll always be my number one because in a club like Man United there must be competition in all positions," he said.