Allegri claims Costa fell for provocation
Costa elbowed, headbutted and spat in the face ofFederico Di Francesco in the closing stages of a game in which Cristiano Ronaldo scored twice to give Juventus the lead.
The winger could serve a three-match ban that would include Juventus' clash with Serie A title rivals Napoli on September 29 but Allegri felt Costa was provoked.
"Maybe he was irritated at a foul before," Allegri told Sky Sport Italia.
"But it doesn't matter, because this absolutely must not happen.
"The one thing we must avoid doing is falling for provocation.
“We could have 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."
90+3' Red card for @douglascosta . #JuveSassuolo [2-1]
— JuventusFC (@juventusfcen) September 16, 2018
Allegri suggested Ronaldo snatched at a number of chances earlier in the game, but said the Portugal superstar's first goal for the club following his transfer from Real Madrid settled his nerves.
"You just had to see the first three games and how close Ronaldo went to scoring," added Allegri.
"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.
"Today, even though it was the first time with this system, we did well. Naturally, the full-backs have to give more width.
"Sassuolo fought hard and as they became tired later on, we took full control of the game."