Manchester United manager Jose Mourinho during the press conference at De Kuip Stadium in Rotterdam, the Netherlands, Sept 14, 2016. [Photo/Agencies]

Manchester United's chances of winning the Premier League could be hindered by their Europa League campaign, manager Jose Mourinho has said.

United face Dutch league leaders Feyenoord in their opening group game in Europe's second-tier competition on Thursday, with Mourinho opting to rest the likes of captain Wayne Rooney, Antonio Valencia and Luke Shaw for the trip to Rotterdam.

The Portuguese, who lifted the trophy with Porto in 2003 when it was the UEFA Cup, said the congested fixture list was not ideal for his side to challenge for their 21st English league title.

"It's more difficult, yes (to win the Premier League when you are playing in the Europa League)," Mourinho told reporters.

"It would be better if we had the privilege of playing on a Monday after a Thursday game to have one day (recovery) but we didn't get that privilege.

"In fact, the gift we have is to play Liverpool on the Monday before, not the Monday after. So (next month) we play Liverpool on the Monday, Fenerbahce on the Thursday and Chelsea on the Sunday.

"So we know that we are not going to have any kind of support on that."

United missed out on a Champions League spot after finishing fifth in the league last season, and the former Chelsea and Real Madrid manager said that the three-time European champions would respect the Europa League.

"It is not a competition that United wants. It is not one I want or the players want but it is a competition where we are and that is the reality," Mourinho added.

"I think you can see by our selection that we come to respect our opponent and we come to win the game. We want to win the competition."