Allegri said: "It's only right that he [Costa] is fined, and he certainly will be".
Juventus, seeking a record-extending eighth successive Serie A title, are the only team still with a flawless record.
"But it doesn't matter, because this absolutely must not happen".
"We tried to dribble past everyone".
The Brazilian's challenge did nothing to prevent Sassuolo scoring - and then it all got much worse.
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Ronaldo has yet to score since his €100 million move from Real Madrid in July, yet Juve have won their first three matches and are top of the table as the league resumes play following the worldwide break.
"He came very close in his first three games", Allegri said at a news conference.
"Football has a way of evening things out and that was the story of the first goal, with the ball bouncing off the post and sitting up for him. We mustn't lose our heads, even if these results destabilise us". Naturally, the full-backs have to give more width.
"It's human to be like that, sometimes we can't control our anxiety", he said.
According a UK Sun report, the eight-year-old has been shipping in goals for the Italian giants' since his switch to the club following his father's £88m switch, from Real Madrid over the summer.
Juventus now turn their attention to the Champions League, which Ronaldo won last season, as they face La Liga outfit in the first game of the group stages.