He doubled his side’s advantage over Sassuolo midway through the second half.
Khouma Babacar halved the deficit in stoppage time but Massimo Allegri’s men held on to maintain their perfect start to the season.
Although Ronaldo had been unable to find the net during the first three league games, Allegri said he was unconcerned about the goal drought despite the star signing’s 23 shots at goal.
"I didn't expect Ronaldo to do more or less than what he did in the first three games," said Allegri.
Ronaldo scored 44 goals for Real Madrid last season including 15 in the Champions League on the way to a third consecutive European title.