The Brazilian was given his marching orders after VAR caught him spitting in the face of Sassuolo midfielder Federico Di Francesco in the 93rd minute with the two having clashed throughout Sunday’s Serie A clash at the Allianz Stadium.
Costa came in for fierce criticism from Allegri following the 2-1 win, which also saw Cristiano Ronaldo break his scoring duck for his new club with both Juventus goals.
“Maybe he was irritated at a foul before, but it doesn’t matter, because this absolutely must not happen. The one thing we must avoid doing is falling for provocation,” Allegri told Sky in Italy.
“We could’ve scored a third goal, but instead we started to play as individuals, we tried to dribble past everyone, this wound up the opposition, who then reacted badly and you get to situations like Douglas Costa.
“You just had to see the first three games and how close Ronaldo went to scoring. Again today, he was a little too hasty and eager to score, but it bounced off the post and right into his path. That unblocked him.”