Argentina ace Aguero left the Vicente Calderon for Eastlands this week in a ?38million deal.
Forlan wishes the 23-year-old well but believes he would have been better off holding out for a move to Andre Villas Boas' Blues instead.
"It was no real surprise to see him leave," Forlan told The People, "but I thought it would have been to a different club.
"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."
Forlan also offers a note of caution for Aguero, having struggled to make an impact himself at United before hitting form in Spain with Villarreal and then Atletico.
The Uruguay ace 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."
Source: Team Talk
Source: Team Talk