John Stones must play regularly in order to fulfil his full potential, according to Manchester City boss Pep Guardiola, who can't put a finger on why the defender continues to struggle for fitness.
Stones has managed just 12 appearances for City across all competitions this season, having picked up a number of niggling injuries which have limited his contribution.
The 25-year-old's latest setback came in a 2-1 defeat to Manchester United at the Etihad Stadium on December 7, as he was forced off the pitch with a hamstring problem.
City had to make do without Stones over the festive period, but he is due back in action when they take on Port Vale in a third-round FA Cup tie on Saturday.
The England centre-back was included in Guardiola's matchday squad for a 2-1 win at home to Everton on New Year's Day, and the Spanish tactician is pleased with how he has progressed since returning to full training.
Guardiola is, however, at a loss to explain why Stones has been unable to stay fit for longer than two or three matches in succession over the past year.
“Step by step he’s getting better, he’s still not at the top but he’s good for some minutes for tomorrow,” the City manager told a press conference. "He has to be fit, after two or three games he fell down and he cannot play.
"We tried to help him, we tried to do the best to help him since the first day of the three and a half years we are here. We do everything for him.
“He has the conditions to do it, but to be the best central defender you have to play one entire season, every three days being there.
“It’s difficult to reach the potential that he has, he has to be at the level to play regularly. The last three seasons and a half he could not do it.
“It’s difficult to know the reason why. Sometimes when you have a tough injury, for example like Aymeric [Laporte] had this season or [Ilkay] Gundogan in the first season, sometimes it’s unlucky, or [Benjamin] Mendy the year he arrived, first year he arrived. That can happen, you know, 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.”
Despite his difficulties in 2019, Guardiola still believes that Stones can reach the high standards expected of him and "become one of the best" given how much time he still has ahead of him at the highest level.
“He’s still young. 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," Guardiola concluded.