"Arsene Wenger is not a small manager".
There was no way back for United and, for once in a troubled season, the Arsenal fans could relax and forget the will he-won't he concerns about whether manager Arsene Wenger will renew his contract.
The Red Devils lost 2-0 to Arsenal at the Emirates this past weekend, and it was another match between Wenger and Mourinho that was not short of drama.
By winning the game, with second-half goals from Granit Xhaka and Danny Welbeck, Arsenal not only reignited their fading hopes of a top-four finish, they also gave themselves real hope of finishing above United once again - a position they have claimed in every season in the post-Ferguson era.
Mourinho said their cramped schedule, which includes a possible two further Europa League games, means they must win the tournament to qualify for next season's Champions League.
Jose Mourinho made eight changes to his United side from Thursday's Europa League semi-final tie at Celta Vigo, ahead of the second leg this week at Old Trafford.
And this is where the Ferguson anniversary carries even greater significance, in that the squad that United now have bears the scars of four years of upheaval and poor decision-making.
"It is the first time I saw them [the Arsenal fans] smile, I saw them enjoy [it] and they deserve [it] because they are an wonderful club and incredible fans [who] also deserve some joy".
Mourinho's team has been hit hard by injury, with Zlatan Ibrahimovic, Marcos Rojo, Ashley Young and Luke Shaw all out for the season, while Marouane Fellaini missed this game because of suspension.
"To play against (Alexis) Sanchez, (Mesut) Ozil, (Danny) Welbeck. these are big players".
Manchester United manager Jose Mourinho gestures during the English Premier League soccer match between Arsenal and Manchester United at the Emirates stadium in London, Sunday, May 7, 2017.
United did manage to extend one sequence, though.
"That's a decision", Mourinho said. They have good possession of the ball, so we need to fully recover and focus to win on Wednesday. "For us, that is now impossible".
Victory for Arsenal ensures that they have won successive home league games against United for the first time since November 2001.
"We are fighting for a Champions League spot through the Europa League, but as well through the Premier League and we came here with this aim but unfortunately we couldn't win".
Indeed, the toughest of Wenger's 21 years at Arsenal could yet have a happy ending.