United were 2-0 up inside 15 minutes through Matheus Cunha and Benjamin Sesko, but imploded after the break to allow Arne Slot's men to level with goals from Dominik Szoboszlai and Cody Gakpo before Mainoo secured the win.

A return to Europe's elite competition after a two-year absence and another big game win furthers Michael Carrick's case to continue as United boss next season.

Mainoo has embodied the upturn in United's fortunes since Carrick took charge in January.

Overlooked by Ruben Amorim, the England midfielder earned a new five-year contract this week and celebrated in style with a cool finish from the edge of the box to edge a wild encounter.

Third-placed United open up a six-point lead over Liverpool and are guaranteed to finish in the top five.

Despite an 11th league defeat of the season, Liverpool likely need only three points from their three remaining games to also secure a place in the Champions League.

Slot's side arrived at Old Trafford depleted without the forward trio of Mohamed Salah, Hugo Ekitike and Alexander Isak through injury.

United sensed blood and were well worthy of their early two-goal lead.

Cunha was afforded two chances to find his range from the edge of the box to open the scoring.

The Brazilian's strike flicked off Alexis MacAllister to give Liverpool's third-choice goalkeeper Freddie Woodman no chance.

The waves of United attack kept coming and Sesko appealed for a penalty after failing to beat Woodman from Bruno Fernandes' precise pass.

Seconds later the Slovenian bundled the ball over the line from Fernandes' header back across goal.

Fernandes was inches away from a third when he blasted Bryan Mbeumo's cross wide.

But the warning signs of what was to come in the second half for United were also there before the break.

Gakpo curled wide on one of the many occasions Liverpool were able to pierce the heart of the Red Devils' midfield.

Sesko was replaced by Amad Diallo at half-time and the Ivorian almost immediately gifted Liverpool a route back into the game.

Szoboszlai pounced on Amad's loose pass and ran from inside his own half unopposed before slotting into the bottom corner.

United were architects of their own downfall again for the equaliser.

This time goalkeeper Senne Lammens' pass was picked off by Mac Allister, who fed Szoboszlai and the Hungarian coolly teed up Gakpo for a tap in.

The unusually low stakes for a game between these clubs showed as both threw caution to the wind in search of a winner.

The home side got it when a poor clearance from MacAllister broke kindly for the charging Mainoo to steer into the bottom corner.

United's first league double over Liverpool since the 2015-16 season completes a remarkable turnaround in the two sides' fortunes from a year ago.

Liverpool finished 14 places and 42 points ahead of their rivals on their way to equalling United's record of 20 English top-flight titles last season.

Carrick has also beaten Arsenal, Manchester City and Chelsea in his short reign and now delivered his primary goal with the riches of the Champions League to make his case hard to ignore.

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