Ronaldo netted his first goals for his new club Juventus this weekend, with the Old Lady maintaining their unbeaten start to the Serie A season.
Cristiano Ronaldo's double - his first goals in Serie A - proved the difference at Allianz Stadium, but it was Costa's very bad conduct in the closing stages that will be remembered.
The five-time Ballon d'Or victor was denied a hat trick by a fine save from Sassuolo goalkeeper Andrea Consigli, while he also mishit a clear opportunity late on.
Cristiano Ronaldo scored 450 goals during his brilliant nine years at the Bernabeu, winning the Champions League three times in a row.
Costa was sent off in stoppage time for gobbing on forward Federico di Francesco, but only after the Brazilian winger had jutted an elbow and aimed a headbutt at his opponent...
After the win, Juventus maintains its Serie A lead with 12 points, having a three-point advantage ahead of second-placed Napoli. There was a bit of anxiety after my move from Real. "Football is like that, what counts is to do well and for the team to win".
'I would like to apologise to all Juventus fans for this misleading reaction in today's game, ' Costa posted on Instagram.
"I wanted to score, I'm really happy, " said Ronaldo, who had missed several good chances in previous games.
Juventus next face Valencia in the Champions League on Wednesday as they look to make a strong challenge in Europe this season, part of the reason Ronaldo was signed in the first place.
And Allegri insisted the five-time Ballon d'Or victor should lift the Federation Internationale de Football Association world player award for a sixth time after his performance for Real Madrid last season. Let us know in the comments section below.
Stephan El Shaarawy and Bryan Cristante put Roma, who lost to AC Milan before the global break, firmly in command at half-time.
"It's human to be like that, sometimes we can't control our anxiety", he said.
But it might offer an explanation as to why Costa lost his head.