Hernanes signed for the Milan club on transfer deadline day last month, arriving from Serie A rivals Lazio for an undisclosed fee on a four-and-a-half-year deal.
The Brazil international has started in Inter's last two games, a 1-0 victory over Sassuolo where he set up the winning goal for Walter Samuel and Saturday's 2-1 victory at Fiorentina.
And Mazzarri is delighted with the impact that the 28-year-old has made.
"He is a player who knows how to deal with pressure and that's what we needed," he said.
"He's the guy who can step up and take the reins of the team, do something important like take a corner just the way you want.
"When we lost our sharpness earlier in the season we started dropping points, especially at home like against Chievo and Catania.
"If we'd had a player like him, he might have been able to win it for us in a moment.
"Take Samuel's goal against Sassuolo for example. He's got the composure and class to make those around him play well too."
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