Forlan’s former Atletico Madrid team-mate left the Vicente Calderon for City earlier this week in a £38million deal after being linked with a string of Europe's leading clubs.
"It was no real surprise to see him leave, but I thought it would have been to a different club," the former Manchester United forward told The People.
"Chelsea's game and squad are more suited to his style.
"And of course I am a Manchester United fan, so his decision to move to City is also painful for me."
The Uruguay international added: "I know City are now one of the big clubs in England, so I wish Sergio luck.
"But I know from my own time in England that the Premier League is not like La Liga.
"The key for him will be how he adapts to the style of the game and also the country.
"He can be a success because he is a star... he can be a king at any club.
"The best asset of Aguero is the number of goals he scores, and when he scores them. He is a killer and could also destroy many defenders with his dribbles.
"Personally, it is bad news to me because I will now miss a big companion in attack at Atletico.
"He is not the tallest, but that is not a problem, and neither is the hardness of the English defenders.
"I do not know if (Carlos) Tevez will stay at City, but a Tevez-Aguero pairing could be sensational for City. For La Liga, this is all bad news, but for the Premier League, it's a success to have such a player."