Martial limped in France’s 2-0 win over Kazakhstan on Sunday after establishing Ousmane Dembele’s goal.
The forward appeared to take his cleats in the grass and twist his knee.
Martial tried to continue, but he was clearly embarrassed by the injury and had to be removed moments later.
With United have a crucial few months ahead of them as they attempt to end the season with a trophy, Fletcher may have to turn to a player he has dubbed ‘a defender’s nightmare’ – Mason Greenwood.
The young striker has been in bad shape so far this season having played mostly away.
Greenwood, who is a deadly finisher with the ability to strike with either foot, can play on the wing or through the middle.
But it looks like he could well be used as a center forward in Martial’s absence, especially given Fletcher’s previous comments about his ability.
In the past, Fletcher has praised the youngster and claimed he believes Greenwood is the club’s best striker.
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“I think he’s a starter in United’s best team,” he said late last year.
“Mason plays on the right side, Anthony Martial in the middle and Marcus Rashford on the left, with Bruno [Fernandes] at n ° 10.
“It’s the best offense and the one that put people on the edge of their seats after the lockdown and produced United’s best form.
“I’m sure Mason’s evolution is to be a No.9. He has world class potential.
“The fact that he can go both ways – few forwards can go naturally on their left foot and naturally on their right foot and finish.
“It’s a defender’s nightmare. ”