Pep Guardiola has hailed the continuing improvement of John Stones at the heart of Manchester City's defence.
The England centre-back has been a stand-out performer in City's superb, unbeaten start to the season in both the Premier League and Champions League.
City have conceded just 11 goals in their opening 17 games in all competitions and kept nine clean sheets.
Stones has featured in all those matches, forming a solid partnership with Nicolas Otamendi in the absence of influential captain Vincent Kompany.
After mixed fortunes in his first season following his £47.5million move from Everton, 23-year-old Stones - who also impressed with England recently - now looks an accomplished figure.
City boss Guardiola said: "We cannot forget that last season was the first time he was able to play three games a week. Everton - they play once a week. Here you have the Champions League, you need a process to settle.
"He became a father and that helped his life to be well organised. He has this amazing quality to understand the game, to be calm, to read the next pass.
"Every game last season he made one or two mistakes and this season that is more irregular.
"He is more mature but he's still young and it depends on him if he wants to become better. Hopefully he still has the desire to improve because there are many things to improve with and without the ball."
City are eight points clear at the top of the Premier League and through to the knockout stages of the Champions League, but Guardiola is still not satisfied and continually demands improvement.
He said: "They are shown the mistakes they do every single game and where they can improve. The moment they don't believe they can be better then that's the moment we crash, to fall down.
"They will do a big mistake when they think we've done enough. When we analyse the results, there is nothing to say, but when you see the games and we have 10 mistakes then there is a reason to work for the future."
On Saturday City will look to extend their club-record winning run of 15 games - without the banned Otamendi - as they travel to Leicester, scene of a 4-2 thrashing last season. After that game a comment from Guardiola that tackling practice does not feature in his training sessions was widely picked up.
He argued that if the team dominated possession, as he would like, tackling would not be overly necessary. It would not preclude players working on this aspect of their game on their own, however.
Guardiola admitted that game was a low point but his philosophy has not changed.
Speaking at his pre-match press conference, he said: "The reality is we didn't lose at Leicester because we didn't tackle. We lost because Leicester were better and we played not good. I never spend time practising tackles."