Mourinho did not say who the club's enemies were but he was clearly angry that his team had to play at Chelsea in the FA Cup quarter-finals on Monday, then Rostov and must next visit Middlesbrough in the Premier League on Sunday.
"We have lots of enemies. Normally the enemies should be Rostov but we have a lot of enemies," Mourinho said after a 1-0 win over the Russians sent United through 2-1 on aggregate.
"It's difficult to play Monday with 10 men (after Ander Herrera was sent off at Chelsea), it's difficult to play now, it's difficult to play 12 o'clock on Sunday.
"A lot of people might say we should have scored more goals (against Rostov). But a lot of things are going against us. The boys are amazing boys. We will probably lose the game on Sunday. Fatigue has a price."
Mourinho explained that Marcos Rojo ate a banana during the match because he was "at the limits."
"The player asks for a banana. It is not to laugh at all. It is to respect the players and their limits, they give everything."
He admitted the United were "afraid" of extra time.
"They (Rostov) are fresh, they had a winter break, they had winter pre-season, they play one match per week, they played against Terek Grozny Sunday afternoon, we played Chelsea Monday night," he said.
"If the game goes to extra time it would be an advantage for them because my players were at the limits. In the first half, we were much better and we should be winning but we weren't and then the game was open until the end."
He added: "I will remember forever when I spoke to the Uefa delegate in Rostov. He told me if any of our players gets injured, the insurance pays. Whoever decided the Monday and Sunday games probably thinks the same way."