It is in the Champions League that his first six months were always going to be judged most markedly, and Tuesday's Old Trafford fixture is the biggest of Sanchez's United career so far as a result.
Tonight's opposition Sevilla have had mixed form since they met United at home and have fallen away from the top four.
"Liverpool is Liverpool, it is a big match", Mourinho said. "But Sevilla and Brighton are "live or die" games so the next two are much more important than this one".
"In football, you are going to be up, you're going to be down".
Marcus Rashford was exceptional at Old Trafford and should start yet again.
Had Alexis Sanchez made a better fist of things in the No 10 role, United might have had even more to show for their solid midfield and defensive foundations, in which the return of centre-half Eric Bailly helped to draw outstanding performances from Chris Smalling and Ashley Young.
United are in a good run of form this season, and they seem to be getting hotter at the ideal time. He gives everything, he has done it for so many years now, we know exactly what he brings and he will have a great time here.
Mark Hughes 'favourite for Southampton job'
Southampton have won only once in their last 17 EPL outings, though the team have qualified for the quarter-finals of the FA Cup . Southampton fired manager Mauricio Pellegrino on Monday in a bid to stop the team sliding out of the English Premier League.
The former Stoke City and Blackburn Rovers midfielder has admitted regret at not completing a deal last summer to join Paris Saint-Germain, who are still licking their wounds following a 5-2 aggregate defeat to the reigning European champions.
And Red Devils boss Mourinho chose to single out Lukaku for special praise after Saturday's important 2-1 victory over Liverpool FC.
Rob is ESPN FC's Manchester United correspondent.
He bade farewell to the Premier League with a thundering long-range strike against Liverpool, in a 6-1 win that was also Steven Gerrard's final game for his boyhood team.
Without an away goal to sustain them, however, United will be vulnerable if Sevilla can finally find a way past the Spanish goalkeeper. That means United must win.
Vincenzo Montella is hoping it is fourth time lucky as Sevilla attempt to overcome Manchester United and reach the Champions League quarterfinals. Sevilla will go through if it ends in a score draw, and another 0-0 will see the tie go to extra time.
The Red Devils failed to break Sevilla's resistance at Sanchez Pizjuan but also kept a clean sheet, and will believe that they can get the work done at Old Trafford on Tuesday.