Jose Mourinho says he believed Manchester United "were always in control" before Feyenoord struck a surprise winner in their Europa League group-stage opener on Tuesday.
The 1-0 away defeat against Feyenoord meant that United have suffered back-to-back defeats, following their home derby loss in the Premier League against Manchester City on Saturday.
After beginning the game with a much-changed lineup, Mourinho made three substitutions shortly after the hour mark, bringing on Zlatan Ibrahimovic, Memphis Depay and Ashley Young, but a United goal still never came.
Anthony Martial, Juan Mata and Marcus Rashford were the players brought off, but the tables were turned when Tonny Vilhena scored the decisive goal for the Eredivisie side in the 79th minute, leaving Mourinho frustrated.
"Of course I am unhappy, especially because the way the game was going in the moment we don't deserve to lose," the manager told BT Sport.
"The second half, the game was even. We were always in control. We were trying to win the game and they were trying not to lose.
"In that moment we were pushing and they capitalised because it is exactly at that moment where we lose the game. We were double unlucky because it's a clear offside.
"The second half was better than the first. We showed more intensity. The first half, my players were in control of the game but not pushing. In the second half, they defended with a great crowd and a great spirit."
Mourinho warned that United are going to have a target on their backs in the Europa League this season.
"We know that we are going to be the team that everybody wants to play against in the Europa League," he said. "We know that everybody wants to fight until their limits against us, which they did. And they had luck to score the goal in the circumstances that we know about."
Marcos Rojo gave a particularly disappointing display, with Mourinho bringing in the Argentine and Matteo Darmian as full-backs, while resting Luke Shaw and Antonio Valencia.
The former Chelsea manager refused to single out players though, saying: "I don't want to go into that direction or personalise. Some players were playing their first minute of the season, some players badder than others, but I can't just judge them after one game."
United next play at Watford in the Premier League on Sunday, and Mourinho confirmed he would return to a more traditional lineup.
"We left Valencia and Shaw back to be fresh so we go back to a normal team obviously without so many changes," he said.