Manchester United manager Ole Gunnar Solskjaer said he will make some "tough decisions" at Old Trafford as he prepares for a squad overhaul.
The Red Devils - who enjoyed a resurgence following Solskjaer's arrival in December but have lost four of their past five games - are sixth in the Premier League and three points adrift of the top four.
Ander Herrera is reportedly set to join Paris Saint-Germain and there are also doubts over Juan Mata, while the futures of Alexis Sanchez, Eric Bailly, Matteo Darmian, Marcos Rojo and Antonio Valencia are uncertain.
Amid links with Borussia Dortmund's Jadon Sancho, West Ham star Declan Rice and Real Madrid forward Gareth Bale, Solskjaer said: "I'm under no illusions that this is not going to be plain sailing, it's going to be hard.
"It's going to be survival of the fittest or survival of the best. 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 and what we do every single day here to make us challenge again.
"Liverpool and City are too far ahead 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, Mike [Phelan] and the club.
"We have a certain standard here. 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.'"
Solskjaer's United will welcome West Ham to Old Trafford on Saturday in the Premier League before travelling to Barcelona for the second leg of their Champions League quarter-final on Tuesday.
United are 1-0 behind in that tie, courtesy of Luke Shaw's own goal in the first leg, with a meeting with either Liverpool or Porto the prize for the winner in the semi-final.