Per Florian Plettenberg, the Premier League side see the 19-year-old’s sale as a potential route to help fund an overhaul in the summer transfer window.
United are reportedly confident that a combination of planned and potential departures will generate the necessary funds for a major summer spending spree.
Mainoo is considered a possible candidate for sale, along with Victor Lindelöf, Casemiro, Christian Eriksen, Marcus Rashford, and Antony.
Chelsea are understood to have a long-standing interest in the central midfielder but could face competition from Bayern Munich in the race for his signature.
The England international has been out of action for much of the season due to various injury issues, his most recent setback coming in the form of a muscle issue sustained in their 2-1 FA Cup win over Leicester City earlier in the season.
Ruben Amorim’s side continue to languish in the bottom half of the table, sitting in 13th with just 37 points from their 29 games.