Aguero has confirmed that he will leave the club when his contract expires at the end of the season, ending a 10-year spell at the Premier League side.
The Argentine striker is City’s record goalscorer and has won four Premier League titles during his time in England, but has been hit by injuries this season and will turn 33 in June.
Richards, who played alongside Aguero during his time City, said he was “devastated” by the news of his upcoming departure, which the former England right-back believes is premature.
“Do I believe he’s still got plenty to give? Of course I do,” Richards said in his column for the Daily Mail. “A fully-fit Aguero, even at that age, can score you 15, 20 goals in a season.
“This is not a striker who is over the hill, so that’s why this decision is proving difficult for me to accept. This is arguably the best-ever striker to grace the Premier League, up there with Alan Shearer and Thierry Henry.
“He hasn’t been pleased with sitting on the substitutes’ bench [this season]. He wants to play, and that’s understandable. Now he and the club are parting ways, and I cannot help but feel sad about it all.”
Richards was team-mates with Aguero at City when the striker scored the goal to win the 2011/12 Premier League title in the final minute of the season.
“If I had to pick someone to score to save my life, it would be Aguero, every single time,” Richards added.
“I owe a lot to him personally. For starters, without this man, I wouldn’t be a Premier League title winner.”
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies