A 1-1 draw with Swansea on Sunday was a sixth stalemate in eight games at Old Trafford and harmed hopes of finishing in the top four.
To make matters worse for United, Luke Shaw and Eric Bailly also suffered game-ending injuries, leaving Mourinho with a major personnel crisis ahead of Thursday's Europa League first leg semi-final visit to Celta Vigo.
The pair joined fellow defenders Chris Smalling and Phil Jones, who have been out since late-March, on the injury list and their absence means United will travel to Spain without a recognised centre-back.
"I don't know about the injuries," said gloomy United manager Mourinho.
"I think Luke Shaw's must be a big injury, because to leave the pitch after 10 minutes I am expecting a very big injury," added the Portuguese boss, who has publicly criticised the left back several times this season.
"I prefer not to speak about Phil Jones and Chris Smalling. I don't think they will play, that is my personal opinion. I prefer to speak about Juan Mata giving everything to be available. I am grateful for that."
Sunday's draw saw United set a new club record of 25 consecutive games without defeat in the same top-flight season.
But it was also the team's 57th game of an exhausting campaign. In comparison, Swansea have played 19 games fewer -- exactly half an entire league season.
"We lost players and we lost points, so yes today was a bad day," said Mourinho, whose side ended the weekend just outside the Champions League places in fifth spot, a point and a position below Manchester City.
"We did not look tired and exhausted, we are tired and exhausted," he insisted.
"You cannot isolate the performance out of the context.
This is the ninth match of April, it is not human. We have a squad of 22 that is reduced to 13 or 14 players. The players are very tired."
Paul Pogba and Marouane Fellaini will return against Vigo, from injury and suspension respectively, and Mourinho will have to explore emergency defensive options which could include veteran midfielder Michael Carrick and untried youngster Axel Tuanzebe.
"I know that the players I choose they give everything," said Mourinho. "So it doesn't matter if I go with (Matteo) Darmian central defender, if I go with Carrick or Axel Tuanzebe.
"I trust the boys, the spirit is amazing, the group is phenomenal. Fellaini and Pogba will be back for that game, so a bit more options and we try with everything we have and we go again," the former Chelsea boss added.