Manchester United are in the middle of an injury crisis and could be without as many as nine players.
United lost to Valencia on Wednesday to cap off their Champions League group stage, missing out on top spot.
But Mourinho is still waiting on players returning to training even with just two days before the next match.
“The only two players the were fit to play but didn’t travel was De Gea and Matic,” he said.
“Apart from that, all of them was not a choice. It was because they were with problems. So we took to Valencia the possible team, apart from De Gea and Matic. Both had a reason to stay.
“De Gea has played every minute and needed a mental rest. Matic has played every minute and needed a physical rest.
“I hope that between now and tomorrow’s training session we can have some positive news. But the situation is not easy.”
Mourinho then spoke to the press about the situation.
He added: “The latest is to tell you again that in Valencia only De Gea and Matic were available to play and didn’t travel by decision apart of that the players that didn’t travel were not fit to play in Valencia, all of them had injuries and they were not ready.
“All of them didn’t train yesterday again and today they will be a little, I don’t want to say test, but a little introduction to training to see the answer, adaptation and to see if we can increase a little bit tomorrow to have them for Sunday.
“But for sure, I hope, is better not to say for sure, I hope some will be available but for sure some will not be.”