The Red Devils extended their winless run to four games and dropped to third in the table following Chelsea’s 5-0 thrashing of Stoke earlier in the day.
To make matters worse, Romelu Lukaku was forced off on a stretcher and needed oxygen and could miss the next two games.
Man Utd‘s clash with Southampton was not short of chances as David De Gea made a magnificent save with his legs to keep out Shane Long.
And Paul Pogba had a late goal ruled out for offside after tapping in the rebound of Anthony Martial’s initial shot.
But it was not the offside which Mourinho had a problem with after the match.
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Instead Mourinho fumed at Pawson for not awarding his side a 33rd-minute penalty after the ball struck Saints defender Maya Yoshida on the forearm in the box.
Yoshida was attempting to snuff out Jesse Lingard when it happen and sparked mass claims from the home side.
When Mourinho was asked if his side had a disappointing afternoon, he instantly diverted the blame to the officials.
“I think it is also a disappointing afternoon for Craig [Pawson, the referee] because he’s a fantastic referee, one of the best in Europe,” Mourinho said.
“We missed easy chances but I’m really unhappy with the penalty, not with Pawson’s performance – Today I think Craig had a good performance, it was a big decision to make from Craig.
“I’m happy with the players’ attitude, the players’ desires and I’m very happy with my place.”
Mourinho has often been punished this season for criticising the referee and has been slapped with fines and bans.
But to avoid further sanction he tried to praise the official while insisting that he got the decision wrong.
Meanwhile, Southampton boss Mauricio Pellegrino was delighted to take a point away from Old Trafford.
“I think we were solid and we had our moments in the game,” he said.
“I think United had moments where they were uncomfortable on the pitch and we played really well.
“I think we can use this performance as an example for the next game.
“Today I think our positioning was good and we did very well.”