Isla, while he isn't particularly great technically, could be key for Juve. He's full of passion, and is a great tackler. I'm not sure where he'll most likely play. He's usually at his best in the centre of midfield with either a holding role or a box-to-box role. The guy has so much energy. With a long season, he could drive the team on, when he's healthy.
I think he'll most likely play as a right midfielder or right back if the team eventually goes back at playing with 4 defenders. It's the same position were he currently plays with Chile's national team and he's basically competing for the spot with Lichtsteiner, who I believe is definitely not better than him when they're both at their peak level. I can only see him playing in the centre if there's some injuries going on since it's already well covered with Vidal, Marchisio, Pirlo and then Pogba waiting for his chance. And, yes, I agree that he could be a key player when healthy, just hoping the knee is 100% fine soon.