The goalkeeper left Espanyol for Barca this week after his €25m buyout clause was paid.
Collet slammed: "It annoys me that he went with the 'monster'. He betrayed his teammates, coach and fans.
"The methods were wrong."
Also harsh was economist and fan Gonzalo Bernardos, who wrote on social media: "Why did you lie to so many people? You had already reached an agreement with Barça, but you kissed the Espanyol crest.
"You went from being an idol to a traitor for so many children."
According to Bernardos, Joan García ruined everything he had built in the last two seasons and could have left "through the front door", but he preferred to "exit through the back door".
The goalkeeper, born in 2001, has signed a contract until 2031 with the Blaugrana.