Everton manager Ronald Koeman is unsure if midfielder Ross Barkley will ever fulfil his potential.
Barkley, 23, has long been touted as England's next Steven Gerrard or Frank Lampard, having come through Everton's youth academy.
However, Koeman is desperate to see more from Barkley, who was lucky not to be sent off for a rash challenge on Jordan Henderson in his side's Merseyside derby loss on Monday.
"First of all I don't like to say because every word I say about Ross is used by the media in a negative or positive way," the Dutchman told UK newspapers.
"Of course I think the player needs to improve. He needs to improve the tactical aspect of his football, out of his position, he needs to be more clinical and have more creativity in the offensive part of the team, but he is working hard on that and trying to get the best out of himself.
"Maybe that type of aggression meant he lost a bit of control in that action [on Henderson] but he is working hard.
"He needs to improve to be the player that maybe everyone expected when he was 18 or 19."
Koeman said there were no guarantees Barkley, a 22-time England international, would deliver on his potential.
The Barcelona great feels some players simply find it too difficult to take the step up.
"I am not sure. I had the same stories in Holland with players who, when they were 16 or 17, they were too good in the academy," Koeman said.
"They didn't really have competition because they were stronger or they were a better player than the rest.
"But then they step up a level and everyone is strong, everybody is a good player, and still you need to make a difference. And that is more difficult."