Pogba was a teenager when he moved, Can isn't. Pogba was partly a product of United's academy, Can was brought in from Leverkusen. Pogba didn't play any football at all in United's first XI, Can has played over a 100 games for our team under two managers.
The similarities are that they leave on a free[if Can leaves]. That's it. Pogba has a heigher ceiling right now than Can does, and he's done more for Juventus, than Can has for us.
I think Can will be a good player, he's inconsistent now. Pogba can be a word class player [not under Mourinho imo].
The point I'm making is at their point of departure, both players (will potentially) leave as enigmas in a way. The fanbase is still quite undecided on Can, some people think he's nothing special, others think he's our best midfielder by far. Pogba, when leaving United, fell into those two brackets within the fanbase as well - despite a small sample size, but a lot had seen him already playing for France's under-19 sides and reserves. He'd only played 7 games for the first team at United, and was part of their academy, but he played 7 games for France during 2013, the year he left United, so he was already a player with a lot of potential, similar to Can - despite Can making a lot more appearances for us. They were held in a very similar regard, I'd say.
Not comparing Can to current day Pogba, but to the Pogba before Juventus. Pogba was admittedly only 19 when he moved, but he'd developed at a ridiculously quick rate. Can's 23 now, and he could well be a 'late-bloomer' as he understands his own game and could surpass Pogba's 'ceiling' during his time at Juventus, if he goes. Who's to say Can doesn't do what Pogba did and show his talents under Allegri's coaching?
Don't really want to bring him into the discussion, but Alonso wasn't considered world class by any stretch of the imagination and fairly inconsistent, similar to Can, during his early seasons at Sociedad, Eibar and Liverpool. Not until 2008/09 did he come into his own and showed just what a freak of a midfielder he was, having made his debut for us in the 04/05 season - so his fifth season. Can's entering his fourth season now so no reason to believe he wouldn't go a couple levels higher like Xabi did for us.