"He's more mature now than when we met at the club … He's 23 and he can be a better player."
Southampton had the smallest consolation of becoming the first team in 630 minutes to score against City, but the form of the leaders is sinister by the rest of the division.
The City is scoring one at a slightly faster pace than them and registering a record of 106 goals last season, and they made a devastating start at Etihad which ended the game as a match within 18 minutes of kickoff.
Five minutes passed when Aguero orchestrated an attack on the edge of the Southampton area, David Siva, then moving the ball to the overlapping Sane, whose cross was turned into his own goal by Wesley Hoedt dos Santos.
It took another six minutes for City to double the lead, Bernardo Silva freed Sterling on the right and the English forward pointed to Aguero from a distance.
It was the 150th goal of the Argentine league for City, a milestone that took him to the same level as Michael Owen in eighth place in the all-time Premier League goalscoring list.
Apparently, the City was able to score almost at will and a smooth move made him do three in the 18 minutes.
Once again, Sterling was heavily involved, accepting Bernardo Silva's pass and putting in a cross that was a little too high for Sane, but he arrived for David Silva, who won on the far post.
Southampton coach Mark Hughes, whose future has been the subject of increasing speculation after a race that has led him to win only four of his 19 league games, has fiercely reacted to an even more shocking defense from his team.
"We were poor in this opening period, you have to be determined to keep a great team in town," Hughes said. "We were a little light and we did not deal with one-on-one situations."
Southampton, however, got a goal back after half an hour, becoming the first English team to score against City from Newcastle at Etihad in early September.
It was a rare error from goalkeeper Ederson, who led to Santos goal, the Brazilian diving unnecessarily at the feet of Danny Ings and tripping the attacker, who hit the penalty kick.
However, there was never a suggestion that Southampton could throw anything that looked like a turnaround and City finished the first half of the game, scoring the score.
David Silva's ball seemed to be coming out of the game, but Aguero showed an impressive persistence to chase and pull back to Sterling, who first missed, but then passed by goalkeeper Alex McCarthy, six yards.
Southampton improved early in the second half, with James Ward-Prowse and Ings – diverted away from Fernandinho – forcing Ederson to make good stops.
But City continued to threaten, and Sane nearly made five with a vicious left-foot shot from the edge of the area that hit the bar just after the hour.
Sterling made it in the 67th minute when he picked Aguero's pass perfectly weighted to score his second goal of the afternoon.
Sane completed an impressive individual and team showing in additions when he made a left kick from 18 yards after another assist from Sterling.