Mainoo, 20, has gone from starting the Euro 2024 final with England to becoming a fourth-choice midfielder at Man United under Ruben Amorim.
He asked to leave the club on loan late in the summer transfer window, but his request was quickly turned down by the club.
The United academy product has since featured for just 183 minutes across his 10 Premier League games, starting none of them.
Speaking to Covers.com, ex-Man Utd star Ince insisted that Mainoo must now leave the club, especially if he wants to push for a place in Thomas Tuchel’s World Cup squad.
"If I were in Mainoo's position, I’d be looking for a change of club in January," he said.
"The World Cup would be my ultimate priority, and leaving Manchester United - permanently or on loan - appears to be the best option to secure a place in the England squad.
“When Mainoo first burst on the scene, everyone was raving about him; he was playing well for Manchester United and was getting minutes for England.
“But since then, something has happened, and I don’t know whether it’s the player or the manager, Ruben Amorim.
“Perhaps his rise happened too quickly, but whatever the reason is, the journey hasn’t been easy for him."