The Juventus goalkeeper did not have to spend €7.2m on a player to become the first Italian to score in the Champions League final, but Guarin did.
The Bianconeri have been criticised for their transfer policy, with some players being used to help the team win, but not enough to earn the €5m the French side paid for the 24-year-old.
Guarin was one of the most influential players in the last Serie A season, with a string of assists to help Juventus win the title and a move to Chelsea this summer.
Juventus are now without their No. 1 goalkeeper for the final because of an ankle injury.
Coach Leonardo Darmian said he was “surprised” to learn of Guarin’s departure, which comes as Juventus prepare to face Borussia Dortmund in the final.
“I was surprised, because I had a conversation with Guarin last week and we agreed that he did not need to buy,” Darmion said.
“He did not sign a contract, but the fact that he’s leaving is a good thing.”
Guarin joined Juve from AC Milan in 2012 for €4.5m, but only made four Serie A appearances in five years, and has since gone on to become a popular target for the Biancones.
The club were reported to have paid €20m for the former Italy international, who was forced to quit after the 2014 World Cup, and he is expected to make a comeback.
“It’s a good time for Juventus, and I’m happy that Guarin will return to the Biancans,” Darmsi said.