He sustained the injury midway through the opening period, and initially tried to run it off. However, he failed to do so, and so coach Niko Kovač was forced into a substitution. Inter Milan wonderkid Mateo Kovačić was the chosen replacement.
As of yet, there's no word on the nature of the injury, though he left the field clutching the top of his left leg. It looked like a muscular injury, which will hopefully mean he's sidelined for weeks at the most.
Croatia have made a pretty impressive start to the match in Milan, and are currently drawing 1-1. Italy took an early lead through Antonio Candreva, though Croatia soon pulled things back courtesy of a goalkeeping error by Gianluigi Buffon, who let an Ivan Perišić effort squirm into the back of the net.