I think it's a move that is a bit surprising to be honest.
When he was bought from Lorient there was a lot of hype surrounding him, as he had shown real glimpses of quality there. The commotion was mostly because Christian Gourcuff who was the Lorient manager at the time, and a coach who has been renowned to develop young players (Gignac, Koscielny, Gameiro, Jallet all played for Gourcuff in early stages of their careers and he gets a lot of credit for developing them) had been extremely incensed at the management for selling Lemina to Marseille at the end of the mercato and for a small sum. Gourcuff thought very highly of him. He didn't break into the Marseille side though, so he probably didn't do as well as he was expected to.
That being said, he does look a decent prospect. Defensively he is very good; he’s more the energetic type than the disciplined one, but he’s an excellent tackler, really strong presence in the middle of the pitch. He’s quite versatile too; last season Bielsa used him as a right wingback in a 3-5-2 formation, when L’OM had a mini injury crisis and he didn’t do too badly.
All in all a gamble for Juve, as he hasn’t really shown enough that would make one expect him to move to a club like Juve, but he’s still young and he does have ability, so it could pay off.