Aguero will bring the curtain down on 10-year stay in the blue half of Manchester at the end of this season after City decided not to renew his expiring contract.
The 32-year-old will depart Manchester City as the club’s all-time leading goalscorer with 257 goals in 384 appearances across all competitions.
"We have still two months ahead, and today I saw the training session, how brilliant he was and his commitment," Guardiola told Sky Sports.
"Sergio is irreplaceable. I could say in terms of numbers he can be replaceable, it's not easy when you see more than 360 games played and more than 250 goals, the amount of titles.
"So it's the legend, the best striker this club has ever had for this century through the years, but is irreplaceable in the souls, in the hearts, in the minds of our fans, of our people, of the players that played alongside him and all the managers who worked with him.
"He's a fantastic person, a human being, and I'm pretty sure if his mind is still aggressive like he is, he can extend his career longer and longer than maybe two or three or four or five years."
Aguero has won four Premier League titles, five league Cups and one FA Cup with Manchester City but fitness issues have restricted him to just 14 appearances this season.