Martial nodded off the line after Carroll met a Payer corner, then was booked for foul on Cheikhou Kouyatein the 71st minute.
And a minute later, he picked up a pass from Rashford, then deceived keeper Darren Randolph at his near post as he cut in from the left to make it four goals in his last three games.
But West Ham levelled in the 77th minute, Dimitri Payet picking out right-back Michail Antonio to nod in at the far post after his free-kick bounced back to him off the wall.
In the 81st minute Payet got his free-kick just right and Reid was able to get in front of Blind to head the winner.
Cue more frustration for van Gaal and a major setback in his hopes of staying for one more season at Old Trafford.
Defensively his team looked so fragile last night, but at least Martial is suggesting he can be some striker.