Both City and Everton have started the season impressively with Koeman's side sitting fifth in the table.
Guardiola, whose City side are top after a run of six wins and one defeat this campaign, said the Dutchman, who was his room mate at Barcelona, was one of the best central defenders of his era.
"Ronald is a big friend of mine," Guardiola told reporters on Friday.
"When Ronald needed water, he'd wake up to water from me. I was younger so I looked up to him.
"He was not just a room mate, we spent a lot of time together and he was so generous. Not just on the pitch but off it too -- I learned a lot from looking at him.
"I think Cruyff bought him to teach us why we needed a central defender like Ronald. He was one of the few guys who I never saw miss an important game. He always played good in the big games. He's one of the best central defenders I've met."
Guardiola said midfielder Kevin De Bruyne is set to return for Saturday's game at the Etihad Stadium. The 25-year-old sustained a hamstring injury last month.
The former Barcelona boss also confirmed that full back Bacary Sagna will be out for three weeks with a hamstring problem.
(Reporting by Ian Rodricks in Bengaluru; Editing by Toby Davis)
