With Inter trailing thanks to Giuseppe Rossi's 60th-minute penalty in Thursday's clash, boss Walter Mazzarri opted to replace the Colombian with striker Mauro Icardi.
And the change paid dividends as Inter rallied to win 2-1 via goals from Esteban Cambiasso and Jonathan at San Siro.
Guarin was met with a series of whistles as he exited the pitch, and he responded by showing his displeasure with a gesture to the crowd.
But the 27-year-old insists that he was merely trying to get the fans to back the team.
Speaking to the Inter Channel, he said: "It wasn't a great moment for me when I left the pitch, but it will help me to become stronger on a personal level.
"I care about the Inter fans. Everyone knows what I do every day for the good of the team.
"I didn't want to offend anyone with the gesture. I wanted to fire up the fans to help the team and not whistle every ball we lost.
"They need to be the 12th man on the pitch for us."
Despite the incident, Guarin was delighted with the three points against a tough Fiorentina side.
"We came up against a strong team that has played well in their recent games," he continued.
"We showed a lot of patience and then had the determination and strength to find the winner after equalising."
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