I mean Perisic has never been a fullback at any point of his long career, and only functioned as a LWB in the very limited context of a dominant side in Serie A where the practical requirements of the role were hardly distinguishable from his natural position as a LW.Right away Ange moved Perisic to LW. He could've done that with Porro but didn't.
Porro is more of a fullback. He is nowhere near as classy on the ball or as precise in his movements as someone like Perisic. But what he does have is the ability to terrorize the top right corner of the pitch with pace and directness and dribbling at players. The system needs that and we lack it elsewhere.
He does have the ability to underlap, I'm sure he'll get an opportunity to play the inverted fullback role. But as we saw with Royal, that's a pretty unique brief. You're an out-and-out midfielder in possession, and then you have to recover to runners in behind coming from positions a defender used to playing nearer the touchline won't be used to.
Shutting down speedy wide attackers is the best part of Royal's game, and you could see how he was struggling with the weird new directions the runs were coming from. Porro is already terrible at that aspect. It's a bit scary. I kinda would like to see what he could offer if just given the mission to damn the torpedoes going forward down the flank.
He's a lot more like Jota than Zinchenko, IMO.