Tonny Vilhena powered home the winner after 79 minutes, but Nicolai Jorgensen appeared offside as he broke down the wing before crossing to set up the only goal of the game. United were lacklustre going forward but Mourinho insisted his side did not deserve to lose.
“Of course I am unhappy, especially because the way the game was going in the moment we don’t deserve to lose,” he said on BT Sport 3. “In the second-half we were always in control, we were trying to win the game and they were trying not to lose the game.
“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.”
Mourinho made eight changes from the side which lost the Manchester derby on Saturday, but the Portuguese did not want to discuss individual performances or comment on whether those players given their chance had passed it up in the defeat.
“I don’t want to go in that direction, I don’t want to personalise it,” he said. “Some of the players were playing their first minutes of the season, there have not been many minutes for them, but obviously some played better than others. I cannot judge them only on one game.”
However, Mourinho said he would revert to a “normal team” for Sunday’s trip to Watford, with the likes of Antonio Valencia and Luke Shaw set to return.
An undercooked and under-performing United had a much-changed line-up but looked cumbersome and unwieldy in Rotterdam. It was hardly the response Jose Mourinho craved after Saturday’s derby defeat at home to Manchester City, with few of the eight changes impressing at the imposing if partially-closed De Kuip.
Anthony Martial’s fizzing strike wide was the best United could muster in a lacklustre first-half that showed little sign of improvement after the break.
Zlatan Ibrahimovic, Memphis Depay and Ashley Young were thrown on as Mourinho attempted to jump start his side, but their introduction could not prevent United’s winless run extending to a sixth European trip.
Instead, they left Holland pointless as the Eredivisie leaders struck a decisive blow as offside calls against Nicolai Jorgensen ignored and his cross was turned home impressively by Vilhena.
Istanbul-based Fenerbahce and Ukraine’s Zorya Luhansk lie in wait over the coming months
Zenit St Petersburg staged a dramatic comeback in their Europa League Group D clash with Maccabi Tel-Aviv, s coring four goals in the final 14 minutes to win 4-3. Maccabi are next up for Dundalk on September 29.
Jones, Karsdorp (Nieuwkoop 90), Botteghin, Kongolo, Van Der Heijden, Kuyt, El Ahmadi, Vilhena, Toornstra, Jorgensen, Berghuis (Basacikoglu 68).
Par Hansson, Vejinovic, Woudenberg, Tapia, Kramer.
de Gea, Darmian, Smalling, Bailly, Rojo, Schneiderlin, Pogba, Ander Herrera, Mata (Young 64), Rashford (Ibrahimovic 63), Martial (Depay 63).
Romero, Carrick, Fosu-Mensah, Fellaini.
Jesus Gil Manzano (Spain).