Sandro plays in attacking midfield and also as a left back and will cost Juventus 20 million euros.
Asamoah has endured a difficult five months after being sidelined by a serious knee injury which ruled him out of the 2015 Africa Cup of Nations with Ghana.
He started pre-season training with Juventus more than two weeks before the group reported.
But the Ghana international is still struggling to regain full fitness.
Asamoah was left out of the Juventus squad that played Borussia Dortmund last week in Switzerland and has been excluded from the trip to Poland to face Lechia Gdansk.