MANCHESTER, England -- Manchester United manager Ole Gunnar Solskjaer has warned his squad he will make "ruthless" decisions as he looks to build a team capable of challenging Manchester City and Liverpool.

Solskjaer will oversee his first summer transfer window at the end of the season and the Norwegian is ready to make big changes at Old Trafford in a bid to mount a serious title challenge for the first time since Sir Alex Ferguson retired in 2013.

"It's going to be survival of the fittest or survival of the best," Solskjaer told a news conference ahead of the Premier League clash with West Ham. "That's how ruthless we have to be.

"We need players here who are going to be able to take us forward. That means culture in the dressing room, what we do every single day here to make us challenge again.

"Liverpool and City are too far head at the moment for what we like. We played against a Barcelona team that sets the standard in European football. We're under no illusion it's going to be hard work and there will be some tough decisions made by me, Mick [Mike Phelan] and the club."

Sources have told ESPN FC that Solskjaer is set to oversee a summer clear out that could involve a number of senior players.

Antonio Valencia will leave after 10 years after the club decided against renewing the Ecuadorian's contract. Ander Herrera is set to leave on a free transfer with the midfielder ready to commit his future to Paris Saint-Germain while Juan Mata's contract is also up in the summer. The Spaniard has been offered a one-year deal but it remains unsigned.

Matteo Darmian, Marcos Rojo, Eric Bailly and Alexis Sanchez are among the other names linked with an exit before next season.

Solskjaer is set to be backed with more than £200 million for new signings, including a centre-back, full-back, a forward and at least one midfielder.

The players have a month left to convince Solskjaer they should remain at Old Trafford next season and the 46-year-old suggested he has learned a lot about his players during a disappointing run of four defeats in the last five games.

"We have a certain standard here," Solskjaer said. "We got a great boost, now [it's] more challenging times with the results and performances.

"You want to see players step up and say now: 'I want to be part of this.' You saw Fred and Scott [McTominay] the other day took the mantle up.

"That's what we expect from all the players. When you win 5-0, or when you're 3-0 up, or when you win five games on the bounce it's easy.

"It's easy to be a footballer. But it's not always easy to be a footballer. You get through tougher times. You'll lose games and your character is revealed."