Jose Mourinho labelled the performance of youngster Ruben Loftus-Cheek "unacceptable" after Chelsea's 1-0 friendly win over Sydney FC on Tuesday.
Loftus-Cheek made his first start for the Premier League champions towards the end of the campaign, impressing Mourinho enough to earn a berth in midfield for the final two games.
His displays promised much and the 19-year-old had been touted for more involvement next season.
However, the Chelsea boss has hit out at his latest display in a post-season friendly in Australia, questioning his efforts after coming on as a substitute and then being taken off 28 minutes later with a back problem.
"Ruben said he had a pain in his back but what I was feeling was that he only had the pain when Sydney had the ball," he said.
"When Chelsea had the ball he was playing very well, but when Sydney had the ball I think he was more impressed by watching [Nemanja] Matic and [John-Obi] Mikel work, instead of him working.
"So, with Ruben, it's one step back in terms of my relationship with him. If he doesn't know what it is to play for me and Chelsea it's one step back.
"He has to learn that at 19 you have to run three times more than the others, you have to play to your limits and not play like a superstar with the ball at your feet because this is not Under-18s.
"The Under-18 competition is too easy for him. He's too good to play in the Under-18s but I don't accept that when he's playing in midfield with Mikel and Matic when we don't have the ball he doesn't press, doesn't have intensity and he's waiting for everybody else to recover the ball.
"I don't accept that in the superstars so how can I accept that in a kid? He has to learn what it takes to play for us."
Captain John Terry backed his manager, and suggested the young midfielder needed to buckle down if he wanted to break into the first team next season.
Terry added: "He's a young boy and needs to take it on the chin. If you want to get into this team you need to be very good and mentally strong.
"The manager picks his times when he digs people out, that's part of his learning curve and he's got time to learn about that now. For him to make the next step doesn't happen easily, so it's going to be hard work for him."