Guarin struck twice in victory over Atalanta on Sunday.
Marcelo Ferreyra told Gianlucadimarzio.com: "It was Mancini who change him. When he arrived at Pinetina he said he wanted to focus a lot on him. The trust will change the performance of a player and this has been so for Guarin.
"There is talk of a project around him and Inter wanted to bet big on my client. We have always believed in the club and the trust he now feels makes him happy, he's enjoying the moment.
"In January, there was never a chance he left. And in fact he has remained calm."
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