The Croatian scored a sensational treble at San Siro, after going 54 appearances without netting for the Nerazzurri following his January 2013 arrival from Dinamo Zagreb.
The Croatia international struck a sensational hat trick in a resounding 6-0 win at San Siro, as the Nerazzurri progressed to the group stage courtesy of an 9-0 aggregate win.
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"Kovacic was rewarded for his perseverance," Mazzarri told Sport Mediaset after watching the 20-year-old score for the first time since joining Inter from Dinamno Zagreb in January 2013. "This lad is growing exactly the way we hoped, is becoming consistent and putting his talents to the service of the team.
"The game was pretty much over at 2-0, then it was almost like a friendly. Stjarnan lost intensity and courage. We started well and I was interested to see how we'd react with our heads and our legs.
"We never lowered our guard and that attitude must remain throughout the season."
Dani Osvaldo also scored his first goal for the Nerazzurri, after joining on loan from Southampton this summer, while there was a double for Mauro Icardi.