The six-time winners of the competition, who have endured a turbulent start to the season under manager Ruben Amorim, fought back from a two-goal deficit but ultimately crumbled under the pressure, marking one of the biggest upsets in recent cup history.

Grimsby took control early, stunning the visitors with a clinical first-half display. In the 22nd minute, Darragh Burns threaded a perfect pass to Charles Vernam, who controlled the ball superbly before rifling a shot past Andre Onana at his near post.

Eight minutes later, the goalkeeper flapped at a cross, allowing former Manchester United youth product Tyrell Warren to tap in the rebound and double the lead.

United looked disjointed, with their defense exposed and midfield lacking bite despite giving a debut start to their 73 million pounds summer signing Benjamin Sesko and recalling Kobbie Mainoo for his first appearance of the campaign.

The second half brought a turnaround amid chaotic weather, including thunder, lightning, and heavy rain. Amorim introduced captain Bruno Fernandes and new arrival Bryan Mbeumo from the bench, injecting some urgency.

Grimsby nearly extended their advantage when Cameron Gardner's goal was harshly ruled out for offside, but Mbeumo leveled the score with a composed low finish, showcasing the quality expected from a Premier League forward.

Then, in the 89th minute, Harry Maguire rose to head home an equalizer, forcing extra time and penalties in a tie that seemed destined for United's redemption.

The shootout turned into a 18-minute ordeal, with both sides converting confidently until Matheus Cunha's effort was saved by Grimsby keeper Christy Pym after a hesitant run-up. Onana made amends for his earlier errors by stopping Clarke Oduor's penalty, but it was Mbeumo who sealed United's fate, striking the crossbar with his decisive kick.

Delirious Grimsby fans stormed the pitch in celebration, turning the Humber-side stadium into a cauldron of joy for the underdogs.

A shell-shocked Amorim apologized profusely to supporters afterward, acknowledging the gravity of the defeat. "I just want to say sorry to our fans," he said. "I have nothing to say anymore about the performance. When everything is so important in our club and you try to do everything. You have come from pre-season, even Arsenal you lose the game but you are there, you are improving and you see the performance, but then reach this kind of game, when everything is so important in our club.

"And when you start a game like this… I think my players are talking really loud about what they want, so I’m really sorry to our fans, I have nothing to say anymore."

He added, "I know the best team won today. The best players lost, because when you are a team you can win any game. It was really fair the football today."

With no European commitments this season following their poor finish last term, the worst for United since 1974, this Carabao Cup elimination leaves the Red Devils focusing solely on the Premier League and FA Cup. They have collected just one point from their opening two league matches, and this embarrassment in the fishing port of Grimsby has intensified scrutiny on Amorim, who replaced Erik ten Hag last season and received significant backing in the transfer market.

In contrast, other Premier League teams advanced comfortably: Brighton and Hove Albion thrashed Oxford United 6-0, while Everton and Fulham secured 2-0 victories over Mansfield Town and Bristol City, respectively.

Grimsby Town stun Man Utd in humiliating Carabao Cup exit