Manchester United manager Jose Mourinho was not a happy man after his side kicked off this season's Europa League campaign with a defeat.
It is United's second defeat in less than seven days after last Saturday's defeat against Manchester City.
They face Watford away from home next before facing Northampton in the Third Round of the League Cup next week.
"Of course I'm not happy with the start," Mourinho explained.
"I think in the second half the game, in my opinion, we were always in control. We were trying to win and they were trying not to lose.
"We felt like they were collapsing a bit physically, they were in trouble and it was that moment in which we lost the game. It was double unlucky because it was a clear offside [for Feyenoord's goal]."
"The second half was better than the first.
"We had more intensity than the first half because in the first my players were in control without pushing to try but the second was different. Feyenoord defended with everything, a great crowd, a great spirit we know that.
"We know that in the Europa League everyone wants to play Man United, everyone wants to fight until their limits, which they did. They had the luck to score in the last part of the game.
"Some of the players were playing their first minutes of the season, not many minutes for them. Obviously some played better than others but I have to understand that some of these players haven't played week after week like the other boys.
"It is obvious that we left players like [Antonio] Valencia and [Luke] Shaw at home to play and to rest so we go back to a normal team without so many changes."