United were cruising to victory thanks to goals from Marouane Fellaini and Romelu Lukaku but Mourinho was left angry at a lack of professionalism before Marcus Rashford’s third.
‘After 2-0 I think everything changed and we stopped to play, stopped to think, stopped to play seriously,’ Mourinho fumed.
‘We stopped to make the right decisions on the pitch and we could put ourselves in trouble.
‘Bad decisions, fantasy football, PlayStation football, tricks and when you stopped to play as a team and when you stopped to play seriously I don’t like and you gamble a little bit.
‘The players probably felt that the game was under control with the 2-0 but football is football and you have to respect your opponent.’
Mourinho claimed his side were ‘under attack’ before the Champions League’s opening night, insisting they do not hide behind injuries, although he had a full squad training at Carrington on Monday.
The Portuguese also reiterated that United are at a severe disadvantage in this competition and they lie behind Europe’s elite after a number of huge scores across the continent.
‘We had yesterday a huge attack on some of our important players and we didn’t cry,’ Mourinho added.
‘For these teams – Real Madrid, Barcelona, Bayern Munich – the Champions League starts in February. Now is just the warm-up.
‘In February, when us English teams are trying to survive after the winter period, they are fresh and ready after this warm-up.
‘We are in the second level. The second level is ‘let’s qualify, let’s make the points’ and if we make the knockout phase let’s enjoy playing against the big guys.’