United started the Premier League clash positively, creating numerous openings but again lacking a clinical touch.
But West Ham took the lead out of thin air in the 27th minute when Benrahma profited from a disastrous De Gea error.
The Algerian forward let rip from range under pressure from Aaron Wan-Bissaka and Victor Lindelof, failing to get any sort of power behind his effort.
And that seemingly caught De Gea off guard, with the Spaniard looking like he made a comfortable save down to his right before the ball rebounded off him into the back of the net.
It killed any momentum United had built in the opening stages and made for another uncomfortable half of football away from home.
United’s backline produced telling reactions to the goal, with Lindelof falling to his knees in disbelief.
But later replays captured that Erik ten Hag and Rashford were just as shocked with the manner of West Ham’s opener.
The Dutch boss bowed his head in disappointment while Rashford wore a stunned look on his face and put his hands on his head.
His first instinct was to turn to Ten Hag as if it wasn’t the first time he’d aired his frustrations over De Gea’s increased error proneness between the sticks.
The Spanish stopper is leading the Premier League Golden Glove race, keeping 15 clean sheets this season.
But doubts over whether he is the correct style of goalkeeper for a Ten Hag side have been compounded by costly mistakes this season.
Before the trip to West Ham, he endured a horror show in United’s 3-0 defeat at Sevilla, which saw them eliminated from the Europa League.
And with De Gea reportedly closing in on a new contract at Old Trafford, the debate over his suitability for the No 1 is bound to rumble on.
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