Fresh from losing 2-1 at home to Pep Guardiola's Manchester City, an undercooked and under-performing United started their Europa League campaign with a whimper.
Uninspired on the attack and unwieldy in possession, Feyenoord eked out a late 1-0 win thanks to Tonny Vilhena's late strike at the imposing if partially-closed De Kuip.
Few of United's eight changes impressed as they failed to win on their European travels for a sixth successive match, compounding Saturday's Premier League loss that brought Mourinho's winning start to a shuddering halt.
"When we won the Community Shield and the three Premier League matches, I was not on the moon," the Portuguese said when asked whether the defeats had underlined the scale of the challenge ahead of him.
"I was not saying that we are a phenomenal team and we are going to destroy every opponent.
"I always said it was a very good start but I was never on the moon
I know that the situation is not click your fingers and everybody is perfect.
"But, to be honest, I think we didn't play phenomenal matches in these two defeats, but in both I think it is a punishment for the team because we deserved more than the results we got."
Like the defeat to City, the United boss was frustrated by the officials after Nicolai Jorgensen was not called offside in the build-up to Vilhena's winner.
Another similarity in the Group A opener was Mourinho's belief that the defeat was undeserved, albeit their performance was not brilliant for the duration.
"When you lose matches, there are no miracles in terms of the players' soul," he said. "You win matches and people are happy and full of confidence
You lose matches and people are sad and feel it
That is the normal nature of football players.
"I am here not to let them be in a negative mood or in a negative because we lost two matches in four days.
"Two matches where we didn't play well for 90 minutes, but two matches where we deserve a better result."
Mourinho dismissed the sweeping changes as reason for the defeat - "I played with enough good players to win the match," he said - and was pleased by the players' reaction to his half-time pep talk.
The Untied boss admits the introductions were not expected to play with the "same intensity, sharpness, quick thinking" as Feyenoord's team given their comparative lack of game-time, although that is not an excuse Paul Pogba can use.
Signed to much fanfare and for a world-record fee, the France midfielder was anonymous for large parts of the encounter in Holland.
"I don't like to individualise too much because I answer and your mate comes with another name," Mourinho said when asked about Pogba
"It's something I don't like.
"I think he was like the team
In the first half they were in control but were playing half pace.
"The second half was when they increased intensity and that was when they conceded the goal."
Feyenoord manager Giovanni van Bronckhorst was as proud as his counterpart was frustrated after upsetting the apple cart and defeating United.
"If you win, you always know it is a good plan but there were moments in the game where we had it tough," the former Arsenal, Rangers and Barcelona player said.
"United upped the tempo and used their physical presence in moments of the game, making it very difficult for us.
"But we knew that if we could keep the ball against Manchester, there are possibilities.
"In the second half you saw that Man United want to win with the tempo and the three substitutions, you sensed that they want to leave her with the win.
"At the right moment we scored the goal - a fantastic goal on the counter-attack.
"After that the spirit came in the team and the determination to be there for the last 15 minutes
We fought hard to get the result and after 90 minutes we did."
Source : PA
Source: PA