José Mourinho has spoken of Manchester United having “lots of enemies”, specifically referencing the number of fixtures the club has to play in a short space of time.

The United manager was speaking after losing Paul Pogba to a hamstring injury in the 1-0 (2-1 on aggregate) Europa League victory over Rostov.

The French midfielder, who joined United for a world record fee last summer, is likely to be out for three weeks. Mourinho believes the injury was a result of fatigue. Another United player, defender Marcos Rojo, was so tired during the game that he asked for a banana to be brought on to the pitch.

“We have lots of enemies,” Mourinho said. “Normally it would just be Rostov but we have a lot more.

“It is difficult to play Monday with 10 men (United lost an FA Cup tie 1-0 to Chelsea after having Ander Herrera sent off), it is difficult to play now, it is difficult to play at 12 o’clock on a Sunday. We will probably lose at Middlesbrough now. Fatigue has a price.”

Mourinho admitted his side had struggled against a well organised Russian team, but said that fatigue had played a part in United’s inability to finish off the Russian side.

“They defend with a block of 10 players and that makes it hard,” he said. “Our plan was to try and kill the game in the first half when we still had energy, but it didn’t happen and in the second half it began to run out. It might have been different had we scored in the first half but the nearest we came was hitting the post.”

Before the match, Mourinho told the BBC that he felt United had made mistakes in the transfer market before his arrival at Old Trafford last summer.

“I found a sad club,” Mourinho said, adding that United had got rid of players he would never have sold and also spent money on players that he would never have bought.

Ángel Di María, Danny Welbeck and Javier Hernández were confirmed as three players he would never have kept had he been in charge.

Mourinho also said it would be impossible to restore United to the level of dominance they enjoyed under Sir Alex Ferguson.

“Forget it,’ he said. “Don’t try to go 10, 20 years ago because it is not possible any more. We are not ready to dominate. We are ready to fight for every game but we are not ready to try and win everything.”