"It didn't go our way because of the two goals we conceded", Liverpool manager Juergen Klopp said.
It is still not certain how much the Lancaster-born 21-year-old, who recently opted to play for Scotland internationally, will live up to his potential.
At the press conference, Mourinho also launched a stinging attack on Dutchman Frank de Boer after the former Crystal Palace coach criticised United's sparse use of Marcus Rashford.
Marcos Rojo, Phil Jones, Ander Herrera and Daley Blind were others absent, although Anthony Martial and Zlatan Ibrahimovic trained ahead of the game.
Sevilla defender Clement Lenglet, meanwhile, believes his side are ready for the Red Devils and warned that United will see the best of the La Liga side if they score first.
The victory left Mourinho's side nine points clear of fifth-placed Chelsea with eight games remaining in the battle for a top-four place and boosted their hopes of finishing as the "best of the rest" behind runaway leaders Manchester City.
The striker has not been credited with any assists
"I am upset with the reaction they had with Scott McTominay - a kid of 20 years old was making all the right decisions and they want him to make the wrong decisions, when he decides to break the intensity of the game", Mourinho said.
United's first-half play was very impressive and they were completely deserving of the lead but Liverpool dominated possession in the second-half. Then Manchester United sat deep and defended their box as if their life depended on it. "I wanted to finish on my own terms". Nemanja Matic struck home a gorgeous half-volley in added time for a remarkably good comeback.
This season, he has played 17 times but increasingly in the bigger games.
Manchester United have turned their form around and how, since the first leg in Seville.
Overall, Liverpool had 14 shots, 3 fewer than their per game average in Premier League, but they had only two shots on target - 4 fewer than their per game average of 6.2.
Irrespective of how one looks at Liverpool's first half performance, it was a shoddy one. That means United must win.
Manchester United go into the Champions League Round of 16 second leg with Sevilla needing goals, following a stalemate at the Ramón Sánchez Pizjuán Stadium three weeks ago.