Pep Guardiola believes it would have been impossible to replace Kyle Walker if the England right-back had left the Premier League champions instead of extending his contract till 2026.
Walker, 33, had been closely linked with a move to Bayern Munich during the transfer window after being left out of Guardiola's starting line-up for the Champions League final in June.
But Walker has been a key figure in the City side that has won five Premier League titles since he joined from Tottenham six years ago.
"Kyle is an important figure for us. Not just the club, the players demanded he could not leave," said Guardiola on Friday.
“I spoke with mates and lots of people and they told me Kyle cannot leave. He has qualities you can’t find across the world.
"He can control the best wingers on the planet, who can do better than him?
"His mind is there and here in the pitch. In terms of genetics, few players I've seen are like him. They are unique."
City are the only Premier League side with a perfect record four games into the season.
But Guardiola's men face arguably their toughest test so far this campaign when they visit high-flying West Ham on Saturday.
And with the return of the Champions League around the corner, Guardiola said City's season now starts for real with two games a week to cope with.
"The season always starts right now, after the first international break, when the transfer window is over," he added.