The 23-year-old joined Manchester City for £38m late last week, but the former Manchester United player believes that the Argentine should have moved elsewhere.
"It was no real surprise to see him leave, but I thought it would have been to a different club," Forlan 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."
Forlan managed 10 goals in 63 appearances for United during a three-year spell at the club.