Louis van Gaal has confirmed the Spaniard will not face Tottenham in tomorrow's Premier League opener as he is not in the right frame of mind.
The 24-year-old wants to quit United to join Real Madrid but the two clubs are not close to agreeing a deal.
De Gea was United's player of the year last season and was a key factor in them securing a return to the top four.
But Mourinho believes the strength of United's squad means the goalkeeper's absence will not be a problem for them.
"I think Man Utd has a fabulous squad," Mourinho said.
"I don't think any player not playing one game or a couple of matches that is going to make any difference.
"I look at them as such a strong squad. I don't see a problem."
De Gea has one year remaining on his contract with United and Old Trafford officials believe they will lose him for nothing in 12 months if he doesn't leave this summer.
His proposed switch to the Santiago Bernabeu has rumbled on all summer and Van Gaal is annoyed the situation still has not been sorted.
He said: "He (De Gea) shall not play. A player is also a human being.
"We shall have to see who starts. He cannot manage that.
"We have problems with De Gea. It is in process. I cannot lead this process.
"I can only watch and observe and that is what I am doing. I am not participating."
