Manchester City boss Pep Guardiola is puzzled by John Stones' fitness problems and says the defender will not fulfil his potential unless he finds a solution.
Stones has missed 19 of City's 31 matches in all competitions this term because of separate muscle injuries, last playing in the 2-1 defeat to Manchester United on December 7.
The England international, a reported £47.5million signing from Everton in August 2016, is in line to make his latest return in Saturday's FA Cup third-round tie against Port Vale.
And Guardiola has challenged Stones to put his injury issues behind him in order to get his career back on track.
"He has the conditions to do it, but to be the best central defender you have to play one entire season, being there every three days," Guardiola said.
"He has to be at the level to play regularly. The last three seasons he could not do it. It's difficult to know the reason why.
"Sometimes when you have a tough injury it's unlucky. But in the case of John I don't know. I don't know why he is not able, we are not able, to be a regular for a long, long period because he's a young player.
"As a player he has his future in front of him, it depends on him. It's in his hands and in his head to become one of the best."
Stones' absence, combined with Vincent Kompany's departure and Aymeric Laporte's long-term lay-off, has left Guardiola short of options at centre-back.
Teenage defender Eric Garcia has featured in City's last three matches, starting the last two, and Guardiola was pleased with the youngster's displays over the festive period.
"Eric is so clever," Guardiola said. "'Everything you say to him, he understands, he doesn't need more time to understand.
"Life is about opportunities. We are delighted with how he has taken his. He's helped us in tough moments."