Kobbie Mainoo celebrated his new deal in style as the homegrown Manchester United star sealed a helter-skelter 3-2 victory against archrivals Liverpool to ensure Champions League qualification.

Sunday's eagerly anticipated Premier League encounter pitted the third-place team in the Premier League standings against No. 4 at Old Trafford, where the Red Devils roared out of the blocks as Matheus Cunha and Benjamin Sesko struck inside the opening 14 minutes.

But United gifted Arne Slot's stumbling side a way back just after halftime, with Liverpool's Dominik Szoboszlai capitalizing on a misplaced pass from United substitute Amad in the 47th minute, before Cody Gakpo levelled the score nine minutes later following a blunder from goalkeeper Senne Lammens.

Play continued at a frenetic pace before academy graduate Mainoo settled things in the 77th minute, slamming home in front of the Stretford End just days after committing his future to the club until 2031.

United's victory gave them the league double over Liverpool for the first time since 2015-16 and secured Champions League football with three games to spare, as interim boss Michael Carrick furthered his case for the long-term role.

Simon Stacpoole/Offside/Offside via Getty Images

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Sir Alex Ferguson was unable to watch Sunday's match after feeling unwell before kickoff and being taken to the hospital.

United fans chanted his name toward the end of a first half that their side began at a breakneck pace.

Cunha, back in the lineup after a hip complaint, saw an initial effort from a half-cleared corner blocked but managed to get another shot away, striking into the ground and off Alexis Mac Allister to spark bedlam in the sixth minute.

United kept up the pressure, and goalie Freddie Woodman got his fingertips to a 14th minute prod on goal by Sesko, but Liverpool's third-choice keeper could not deny him for long.

United kept the move alive and Luke Shaw's cross was headed back across goal by Bruno Fernandes, with Woodman only able to push into the bundling striker.

Sesko wheeled off in celebration and further cheers greeted the award of the goal following a lengthy VAR check into a potential handball by the Slovenia striker.

Liverpool showed some fight, from Ibrahima Konaté knocking Sesko into the advertising hoardings to Gakpo slamming narrowly wide, but Slot's side still looked disjointed.

Fernandes fired just wide as United pushed for another before the break, after which a limping Sesko did not return.

Amad replaced him and saw his lax pass cut out by Szoboszlai just inside his own half inside two minutes of the restart, with the Liverpool midfielder confidently running at Harry Maguire before cutting away and scoring a low, left-footed shot.

The United defender headed narrowly wide as they sought to restore their two-goal cushion, while Bryan Mbeumo saw a superb flick from Fernandes' pass hit the foot of the post before offside was called.

Liverpool showed weakness but were handed a 56th-minute equalizer.

Mac Allister read Lammens' woeful pass out from the back and fed to Szoboszlai, who played a quick ball to Gakpo to score.

Play swung from end to end as fans traded chants, some more unsavory than others, with Lammens forced to clear off the line before Casemiro planted a free header from a free kick straight at Woodman.

The decisive goal arrived in the 77th minute in front of an elated Stretford End. Liverpool failed to effectively clear a cross, and Mainoo ran onto the loose ball on the edge of the box, avoiding Szoboszlai's sliding challenge to slam it into the bottom-left corner.

Substitute Rio Ngumoha looked lively and whistled a shot narrowly wide, and Lammens dealt with a deflected cross to help United hold on for the win.