Manchester United players face a "survival of the fittest" to remain at the club, according to manager Ole Gunnar Solskjaer.
Solskjaer was named permanent boss in March after replacing Jose Mourinho in December.
United have won 15 games in all competitions under the Norwegian - but remain in sixth place in the Premier League, two points behind Arsenal.
"We need players here who are going to be able to take us forward," he said.
Asked about his squad, Solskjaer added: "It's going to be survival of the fittest or survival of the best. That's how ruthless we have to be."
Solskjaer suggested that Wednesday's 1-0 home defeat by Barcelona, in the first leg of their Champions League quarter-final, showed how far United have to go to match the very best.
"We played against a Barcelona team that sets the standard in European football," the 46-year-old told a news conference on Friday.
"We're under no illusion it's going to be hard work and there will be some tough decisions made by me, Mick [Phelan, the assistant manager] and the club.
"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."
United's squad is set to change over the close season. Contract talks with midfielder Ander Herrera are ongoing amid reports linking him with a move to Paris St-Germain in the summer, but Solskjaer confirmed Antonio Valencia will definitely be leaving and the club will be looking for a new captain.
Considering who might take on the role, the manager said: "We've had Antonio, he's going. We've had Ashley [Young], we've had Paul [Pogba] and David [de Gea]. Chris [Smalling] has been captain. There are players here who can be captain on the pitch. We just need to decide who's going to be the voice in and around."
United host West Ham in the Premier League on Saturday.