The Brazil international will conclude the 2019-20 campaign at Camp Nou before moving to Juve from next season. The transfer could be worth a further €10m in variables.
Miralem Pjanic is widely expected to complete a move in the opposite direction after both players were reported to have passed their medicals on Sunday.
Quique Setien included the former Gremio player in his squad to face Atletico Madrid on Tuesday, despite the deal being close to completion.
"I have no doubt about his greatest commitment until the last day he is here," the head coach told the media.
"He is a player until the last game of the season. He has committed himself to being one more [member of the squad].
"He will dedicate himself fully to achieving the objectives. I have no doubts about him.
"He has said that he will be in the best conditions. I'll decide if he plays more or less."