According to RMC Sport's Mohamed Bouhafsi (h/t Sport Witness' Tom Coast), Martial recently turned down the offer of a five-year deal at Old Trafford because he feels he is not playing enough, and he is "unable to support his current situation any longer."
Meanwhile, Miguel Delaney of The Independent reported Martial would like to stay, but sources at the club believe manager Jose Mourinho could let him leave as he brings in attacking reinforcements this summer amid interest from Paris Saint-Germain, Atletico Madrid and Juventus.
According to Bouhafsi (h/t Coast), Martial has a "rather cold" relationship with the Portuguese, while Delaney added the coach considered selling him last summer because "he has been frustrated with a perceived lack of application in the striker, and a difference in how they view the role."
The 22-year-old has struggled for consistency this season and has only completed 90 minutes on seven occasions, though he has nevertheless managed to contribute 11 goals and 10 assists.
He was given a combined total of just 23 minutes against Sevilla in the UEFA Champions League as United exited in the last 16, and he has been an unused substitute in their last two matches.
When he has played following Alexis Sanchez's arrival in January, it has often been on the right in order to accommodate the Chilean on the left.
Football writers Liam Canning and Jordan Clarke believe selling him would be a mistake:
Martial is capable of great things, as United recently reminded Twitter:
Though the youngster has struggled to consistently perform at the high level he is capable of during his time at Old Trafford, it remains clear he has enormous potential.
He can still be an invaluable asset to the Red Devils if Mourinho can get the best out of him—the manager would perhaps be better served doing his best to achieve that rather than bringing in someone new.
If he does opt to sell Martial, Delaney added United would want a figure in the region of £60 million for him.
That could be enough for them to bring in a strong replacement, but finding another of Martial's potential will be difficult.