I was keeper for more than 10 years. He didn't come good behin the ball, but look at his positioning at the begin. He must from the middle to the right corner. Perisic strike was so powerfull that he had not the time to come good behind the ball. Commentator said also that it wasn't a mistake or something. A worldclass keeper would have this, but most of mid- and low-table teams have not keepers that would have this ball.
Edit: watched the goal again, his reactions to go to the right post were indeed a bit slow, that is the reason that his positioning wasn't good and can't came behind the ball.
ON: Outside post Verona!