The 23-year-old Netherlands international has battled multiple injuries during his United stay and has missed nearly 11 months of action between 2019 and 2020 with cruciate ligament damage.
With his contract expiring in the summer, manager Ole Gunnar Solskjaer recently acknowledged that he could be on the move this month.
It is now reported that Leverkusen are on course to sign him in a deal worth £1.5m.
Solskjaer said recently “I’m not sure if he will stay or if he’s going to take an option now,” Solskjaer said. “He’s not played enough, so of course he’s been allowed to speak to clubs with a view to leaving now in January.
“It’s difficult when you have good players, talented players, and you can’t give them game-time. He’s now at the stage where he needs to go and play again.
“It might be end of January or in the summer where he finds somewhere else, he’s working hard so he’s ready for a move for any club he feels ready for.”
United signed Fosu-Mensah from the Ajax academy in 2014 and, although typically a midfielder, Fosu-Mensah came on at left-back for his debut aged 18 against Arsenal in February 2016. He made his full debut at center-back against Watford three days later.
The Dutchman spent 2017-18 on loan at Crystal Palace and was loaned to Fulham the following season.