The 21-year-old midfielder finished first time from the edge of the box in the 77th minute to complete a remarkable individual week — he signed a new contract until 2031 on Thursday — and send Old Trafford into scenes of near-delirium. Liverpool had levelled from 2-0 down through Dominik Szoboszlai and Cody Gakpo before Mainoo's composure under pressure decided the contest.
Speaking to Sky Sports after the final whistle, Mainoo kept his message brief and focused.
"We're just looking forward. We still need to finish the season strongly. This is not enough for us. We just want to keep on winning, winning, winning."
He was generous in his praise of Carrick, whose arrival in January marked the turning point of Mainoo's entire season. The youngster had not started a single Premier League match under Ruben Amorim before the change in management.
"He's played a huge part in it. The confidence he puts in, not just me, but all the players. You want to follow him, you want to fight for him, you want to die for him on the pitch. I feel like that's what we've shown today."
The goal carried additional personal resonance. Mainoo scored his first ever senior goal at Old Trafford against Liverpool more than two years ago — at the same end, against the same opponent — and was visibly moved by the connection between then and now.
"I've not been around the goals too much. But I'm happy to be around it in this game, in such a special occasion and such a special fixture."
Among the crowd watching from the stands were members of his family, seated in the same section where Mainoo himself once sat as a young Academy player when the club distributed tickets to its youth teams.
"I saw a couple of my family members. It's a blessing to be able to play here, to be able to score here. It's unbelievable. I used to sit there when the Academy used to give us tickets — I used to sit there as well. So now I'm just so blessed to be in this position. I used to dream of times like this, so my younger self would have been over the moon."
Mainoo was named man of the match. He has now started twelve of Carrick's thirteen league games and his recall to the England squad under Thomas Tuchel followed directly from that opportunity. His new contract makes him one of the highest-paid players in United's squad.
Manchester United sit third in the Premier League with three games remaining, facing Brighton and Hove Albion, Aston Villa and Manchester City.