Manchester United manager Jose Mourinho expects seven teams to be in the running for the Premier League title in 2017-18.

The Red Devils finished in sixth place in the table in Mourinho's first term in charge as they effectively surrendered their remaining domestic fixtures in order to prioritise winning the Europa League.

United have invested heavily on the signings of Romelu Lukaku, Nemanja Matic and Victor Lindelof in the transfer window, while champions Chelsea, Manchester City, Liverpool and Arsenal have also spent large sums.

But Mourinho believes there are as many as seven candidates to challenge for Chelsea's crown in the coming campaign.

"The Premier League is what you all know," he told a news conference ahead of United's opening match against West Ham on Sunday. "I would say seven teams fight for the title and 20 to fight for victory every match.

"It's different to any one of the other leagues. Every opponent wants to win every match, every opponent gives you a hard time. But we are involved in four competitions. Step by step, we're going to fight.

"It's the objective of every one of the top teams and every one of the top team managers."

United triumphed 2-0 over Ajax in the Europa League final to claim the trophy for the first time and secure a return to the Champions League.

Mourinho admits it was a challenge to keep his players fired up to compete for Europe's secondary competition but he does not expect any problems this year.

"Europa League is not Champions League," he said. "It's obviously more difficult to win the Champions League, also because the motivation is higher. My hard work to motivate people to play in the Europa League with ambition and responsibility was hard. It's not hard this season to motivate them to that level.

"We have more experience of working together than we had last season. We have confidence levels of a team that won a European competition and is back in the Champions League. I'm really happy to start the Premier League this weekend because our pre-season was very complete and was a pre-season organised in a way where we start playing MLS teams and ended playing against the best teams in Europe."

Mourinho, who confirmed that Marcos Rojo, Luke Shaw and Ashley Young are still sidelined with injury, believes his squad is in similar shape to how it was at the end of 20116-17, despite their three marquee signings.

"We have [bought] three players, as you know. But for the beginning of the season, we lost important players, too [to injury]. We lost Marcos Rojo, Ashley Young, Luke Shaw, Zlatan Ibrahimovic," he said. 

"So between the players we lost and the players we got, I think we are in a similar situation."