The Portugal forward has been struggling with a knee problem, which saw him miss last weekend's contest with Atalanta before he was restored to face Atletico Madrid in the Champions League.
Prior to the international break, Ronaldo had been hauled off in wins over Lokomotiv Moscow and Milan despite Juve having been level at the time of his withdrawal.
Ronaldo netted a 68th-minute penalty at Allianz Stadium to earn a point against Sassuolo on Sunday, though, and Sarri was encouraged by what he saw.
"The feeling is that he has regained strength," Sarri told DAZN. "He is doing better in the final [moments].
"The next step will be to recover the brilliance."
Prior to Ronaldo's intervention, Juve appeared destined to pay the price for a lacklustre showing that saw Leonardo Bonucci's opener cancelled out as Sassuolo scored through Jeremie Boga and Francesco Caputo.
The second of those was a tame effort that squirmed through veteran goalkeeper Gianluigi Buffon after a rash clearance from Matthijs de Ligt.
"It was an incredible series of mistakes, from a bad back-pass to another that hit a Sassuolo player," Sarri added.
"When you aren't fully focused, a howler can happen."
Sassuolo were playing with teenage debutant Stefano Turati in goal and the 18-year-old put on a fine display in Turin.
After the game, Turati explained he had an important phone call to make.
"I honestly don't have the words! So many emotions, it all happened so quickly and I never expected this. It was an extraordinary sensation and an incredible day for me," he said.
"The save on Ronaldo was the most important for the game, with that free-kick, but the breakthrough for me was early on to stop their counter-attack.
"It was an honor to play against Gigi Buffon, as I used to watch him when I was growing up. I am so happy and shocked that I don’t even know what to say. The first thing I have to do now is call my mum!"