Mourinho identified winning the UEFA Europa League as more important than finishing fourth to qualify for next season's UEFA Champions League.
The result left United four points adrift of fourth place with three games left. And Mourinho ruled out finishing fourth altogether, per James Olley of the London Evening Standard:
Instead, Mourinho has named winning the Europa League as more important for United, per football writer Ben Dinnery:
It's tough to fault Mourinho's logic on one level, especially since United are in the semi-final of the Europa League. The Red Devils have a 1-0 lead over La Liga's Celta Vigo ahead of the second leg at Old Trafford on Thursday.
With the away-goal advantage, United have one foot in the final. It's no doubt why Mourinho rotated his squad so heavily for the trip to Arsenal.
However, the United manager's reference to needing to rest players, per Dinnery, is unconvincing on closer inspection. Mourinho may have changed his side for domestic action, but his starting XI against the Gunners was still loaded with experienced and talented internationals, per Sky Sports Statto:
Having Spain No. 1 David De Gea in goal, fellow Spain international Juan Mata in midfield and United's all-time record goalscorer Wayne Rooney in attack hardly made this a weakened team. Nor did the inclusion of first-choice holding midfielder Michael Carrick, Ander Herrera and England international defenders Chris Smalling and Phil Jones.
There was also Anthony Martial up top, a striker United signed in 2015 in a deal that could eventually cost as much as £58 million.
The idea United were stretched thin by their demands on two fronts was shown up when the Daily Mirror's Andy Dunn tweeted the cost of the two squads in action at the Emirates:
On balance, Mourinho is probably right to prioritise the Europa League as a route into Europe's more prestigious competition next season. With Ajax and Olympique Lyonnais contesting the other semi-final, United have to be strong favourites to win the tournament.
However, Mourinho may have given up on United's top-four hopes prematurely. After all, had they beaten Arsenal, United would have been just a point behind City in fourth. The Red Devils are still only five points behind third-placed Liverpool and have played a game less than their bitter rivals.
There's also the argument clubs with squads and financial resources the size of United's should be able to secure both a top-four slot and trophy success on other fronts.
There is likely to be little sympathy from Mourinho's detractors if his Europa League gamble doesn't pay off.