Mainoo, who signed a long-term contract earlier in the week after being repeatedly snubbed by Ruben Amorim earlier in the season, snatched back a victory for Michael Carrick's side which had slipped from their grasp after a dream first half.
The game kicked off amid concerning news that Sir Alex Ferguson had been rushed from Old Trafford to hospital feeling unwell although there was good news as the admission was described as a precaution and the legendary manager was expected to be soon recovering back at his home.
On the pitch United went straight for their arch rivals and after a bright start they grabbed the lead in the sixth minute when Matheus Cunha's second attempt at a strike from outside the area following a corner cannoned off the body of Alexis Mac Allister and into the net.
The Red Devils survived wayward efforts from Florian Wirtz and Cody Gakpo and doubled their lead in the 14th minute with a breathless counterattack which ended in Liverpool 'keeper saving Bruno Fernandes' header but palming it straight on to the chest of Benjamin Sesko, who bundled it over the line. The goal was reviewed for skimming off the striker's finger but was still given.
But a seemingly certain United win was knocked off course when Amad Diallo, who replaced Sesko at half-time, naively gave the ball away and Dominik Szoboszlai waltzed towards goal from the halfway line to pull a goal back two minutes into the second period. Liverpool were level nine minutes later thanks to another act of stupidity, this time involving a botched pass from Lammens following a goal kick which Szoboszlai scooped up and laid off to Cody Gakpo to score.
Liverpool looked more likely to go on and take all three points but United continued to look dangerous on the counterattack and after Mbeumo had missed a couple of efforts Mainoo took on the responsibility himself.
GOAL rates Man United's players from Old Trafford...
