The likes of Marcus Rashford, Antonio Valencia, Ander Herrera, Phil Jones, Marcos Rojo, Marouane Fellaini and Zlatan Ibrahimovic were all sidelined for Saturday's FA Cup win over Huddersfield Town, while Paul Pogba was forced to pull out on the morning of the match due to illness.
Pogba has since returned to training, though, and Mourinho is hopeful of having three other players available again for the first leg of their last-16 tie in Spain.
"I didn't rest one single player, so we had every player available here. I brought two kids (Ethan Hamilton and Angel Gomes) that played 90 minutes yesterday [for the U23s] because I don't have another player," Mourinho told reporters after his side's win at the John Smith's Stadium.
"Can we recover some of them for Wednesday? I believe so. Rashford, Herrera, Valencia, I think they have a chance. Paul, I don't know. Marcos Rojo, Phil Jones, Marouane, Zlatan - I don't think they have any chance, so we arrive at this crucial moment with some problems.
"But I think mentality is very important. And we have a strong team, strong bonding, strong team mentality. We know it is a very difficult match - but we go for it."
United are bidding to reach the quarter-finals of the Champions League for the first time since 2013-14.