Tactical or not,
Perreyra would have offered everything Sturaro did and would be a danger on the counter. My opinion is he's a Nocerino in the making...we'll see.
Perreyra is more offensive and sprints more on the counters, Sturaro is different with different characteristics, he covers more and was instructed not to attack, Allegri also said he had in plan before kick-off to put Perreyara in the 2nd half and hit Real on the counter attacks.
Just like you I wasn't totally confident when I saw the lineups but I can't question Allegri's choice since Sturaro did what he was asked to do and more if you ask me.
That save was dumb luck. James headed the ball and it somehow deflected on Sturaro's foot and ended up at the crossbar. His foot could have just as easily ended on James' head. That was a high foot and we're lucky the ref didn't signal a penalty. It ended up great for us, but he had more luck than brains.
Probably luck, but credits to him, had the balls to stop this or else this match would have been pretty much over.