Speaking at Daytona last weekend, where Ferrari was celebrating the end of its 2016 motorsport season with its traditional Finali Mondiali event, Marchionne admitted to have been very surprised by the German driver's announcement.
"I actually complimented Rosberg for having had the courage for what he did," Marchionne told Motorsport.com.
"I think timing in life is everything. He's exiting the F1 season as a champion. He can't get better than that.
"I think Nico had reached an age where he'd invested a long part of his life to racing. And coming out as a champion is a culmination of his expectations."
While the Italian executive confessed that he wasn't sure what impact if any Rosberg's retreat would have on Ferrari's arch rival, he scrapped speculation that the move could eventually weaken Mercedes.
"I do not expect a weaker Mercedes at all. Let's not forget that they still have Lewis Hamilton on board, and I'm sure they'll choose a driver of equal potential calibre as they've had in the past."