Jose Mourinho has admired Willian for some time, bringing him to Chelsea in 2013, and the club made inquires regarding his situation at Stamford Bridge in May.
Martial, meanwhile, looks destined for a move away to Old Trafford after his agent confirmed last month that his client wanted to leave Man United, who will consequently need to sign a replacement.
And according to the Telegraph‘s Matt Law, Chelsea are open to selling Willian to United but only if they receive Martial in a swap deal.
The Stamford Bridge club operate their transfers on a one-out-one-in basis to keep the books balanced and have therefore expressed a preference for dealing with Barcelona, who also want Willian and would fund their bid to sign Aleksandr Golovin from CSKA Moscow.
This would be a notably easier route than selling Willian to United given their reluctance to sell Martial – especially to a rival club. You suspect Chelsea will, in turn, place emphasis on getting a good fee from Barcelona before moving for Golovin – a player they have wanted for some time.