Some Manchester United players "need a reality check" after a poor run of form, says boss Ole Gunnar Solskjaer.
United have lost five of their past seven games in all competitions and were knocked out of the Champions League by Barcelona on Tuesday.
They are sixth in the Premier League, two points behind fourth-placed Arsenal, with five matches remaining.
"Most of the players have really impressed me with their attitude," said Solskjaer.
"But of course there are some that need to get a reality check on where they're at."
Asked if forward Anthony Martial, who has scored three goals in his past 15 appearances, was one of those players, Solskjaer said: "I've spoken to individuals - and Anthony is one I've spoken to - and they know what we expect.
"He has a massive talent, he signed a new deal and he knows we believe in him, so he needs to keep on working.
"Anyone that plays for Manchester United has to deliver to stay in the squad and Anthony is not different to the other forwards like Marcus Rashford, Romelu Lukaku and Alexis Sanchez we have to choose from."
As well as two defeats by Barcelona, United have been beaten in the Premier League and FA Cup by Wolves and lost to Arsenal in the league last month.
Solskjaer, appointed permanent boss last month after an impressive spell as interim, said the club will make signings this summer but "have to be realistic" because there is "no quick fix" to challenge for trophies.
"There will be signings but the players have got to come back knowing what's expected of them as well on the first day of pre-season," he added.
"It's not like the 90s where you can arrive overweight and start working from there. It will be great to see who is coming back ready."
Solskjaer's predecessor, Jose Mourinho, complained about a lack of signings last summer, but Solskjaer said United will compete in the transfer marker this year.
"I don't know if all of the critics know what's happening. We've got scouts scouring the planet for good footballers and we'll sign players we think are the right fit - personality-wise, quality-wise, price-wise," he said.
"I'm confident we're doing what we can to get the right players in."
United face Everton in the league at Goodison Park on Sunday before hosting rivals Manchester City and facing Chelsea as they seek a top-four finish to secure Champions League football next season.
Solskjaer said his side's focus "changed straight away" after the 3-0 aggregate defeat by Barcelona at the Nou Camp in the Champions League quarter-finals.
"When you're playing in big stadiums, you've got to step up, because there is absolutely no hiding place," he said.
"We're working in training to create a culture - we want players to take ownership over what is happening here.
"But of course it's the performances on the pitch that will determine where they are at in their careers."