Martial is said to be seeking a move away from Old Trafford during the summer window due to starting fewer than half of the Red Devils' Premier League fixtures this term.
Willian, meanwhile, has thrived at Stamford Bridge since joining from Anzhi Makhachkala in August 2013, winning two Premier League titles and making upwards of 230 appearances for the Blues thus far.
According to The Mirror, United boss Jose Mourinho is desperate to reunite with the Brazil international, while also being frustrated with Martial's purported lack of progress at Old Trafford.
The report goes on to claim that United would want £40m in cash from Chelsea as Mourinho values Martial at £80m in current market conditions, with Willian worth £40m.
Willian is contracted to Chelsea until 2020, while Martial's current United deal runs until 2019 with the option of a further year on the club's side.