United was a shell of the side that so confidently defeated the Turkish outfit 4-1 at Old Trafford a fortnight ago and the Red Devils were behind as early as the second minute thanks to a special overhead kick from Moussa Sow, with Jeremain Lens' excellent free-kick extending their lead shortly before the hour.

A stunning long-range effort from Wayne Rooney reduced the arrears in the Group A contest in Istanbul, but United manager Mourinho conceded Fener was a worthy victor.

"I think they deserved to win. Football is not just about quality, it's also about effort, commitment, it's about playing to the limits, giving everything," he told BT Sport. "So, I think they deserved [to win]. A team that concedes a goal after two minutes is a team that is not ready, not mentally prepared, not concentrated, not focused. 

"In an atmosphere like this to concede a goal and give an advantage to an opponent, then the opponent is where they want to be. They defend, counter-attack, stop the game, simulate free-kicks, put pressure on the referee's decisions. 

"But it's their game and it's our fault we let them be in that situation. After their second goal we had a reaction [that] probably we could get a different result, but I think we didn't deserve more because football is 90 minutes and is to be played in the maximum of your desire, capacity, concentration. 

"So I think they deserved [to win]."

Feyenoord drew 1-1 at Zorya in the pool's other fixture meaning United is third, but just one point adrift of joint-leader Fener and Feyenoord.

Mourinho believes his United players did not approach the game in the correct manner, though he insists qualification for the knockout stages remains in their own hands with two games to go.

"[It was like] they were playing a Champions League final and we were playing a summer friendly," he added to MUTV. "It [qualification] depends on us. If we win our two matches we qualify."