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.
The win is Juventus' fourth from four games this season.
"I wanted to score, I'm really happy", said Ronaldo, who had missed several good chances in previous games.
"I don't expect Ronaldo to do more or less than what he did in the first three games", said Allegri.
Ronaldo's luck changed when Sassuolo defender Gian Marco Ferrari headed a corner onto his own post and the rebound left Ronaldo with the easiest of tap-ins.
The five-time Ballon d'Or victor was denied a hat-trick by a fine save from Sassuolo goalkeeper Andrea Consigli, while he also miss-hit a clear opportunity late on.
Juventus, which is seeking a record-extending eighth successive Serie A title, is the only team still with a ideal record.
Brazilian winger Costa already looked to have lost his composure after aiming an elbow and a headbutt moments earlier but the worst was to come as he spat directly at Di Francesco from inches away.
Winless Chievo are on minus one point after having three deducted on Thursday by the Italian federation over accounting irregularities.
The joy of Ronaldo getting off the mark was overshadowed by the sending off of Douglas Costa late on.
Both sides have made ideal starts to their domestic campaigns with five wins, PSG thrashing St Etienne 4-0 at home on Friday and Liverpool enjoying a convincing 2-1 victory at Tottenham Hotspur.
Inter Milan slipped to 14th with a shock 1-0 home defeat by Parma yesterday ahead of their European opener against Tottenham at the San Siro.