Guarin's stunning late strike at San Siro on Thursday ensured Inter beat 10-man Celtic 1-0 in the UEFA Europa League to progress to the round of 16.
The Colombia international has now scored four goals in his last five games, but he may now be plying his trade in Turkey if Mancini had his way during his short reign at Galatasaray.
"I called him because I knew that he was a good player and I thought he could be very useful for us," the Inter coach said.
"He wanted to stay at Inter and I'm happy about that now – he is playing very well here."
Guarin won the Europa League with Porto four years ago, but the 28-year-old will now put thoughts of European success to one side as Inter attempt to secure a fourth successive Serie A win at Fiorentina on Sunday.
He said: "We have to take it one game at a time, starting with our next outing in the league, which is very important,
"Things are changing [at Inter] – and most of all, the results are changing too."
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.