The Portuguese legend converted a penalty in Juve's 2-0 win over Bologna but Sarri suggested that Ronaldo needs to improve his fitness.
The 61-year-old coach said: "I spoke to Cristiano again yesterday, alone and for a long time. He mustn't lose his confidence.
"I hope that the fantastic player of old will come back. It's just that he's not physically at his best right now."
The victory left Juventus four points clear in the delayed Serie A season and Sarri warned fans not to expect great football for the remaining matches as teams return following the coronavirus shutdown, as well as matches being played in the summer heat.
The former Chelsea manager explained: "It's a completely anomalous situation and there are no points of reference, so it's very easy to make a mistake with the physical preparation.
"You have teams in a physical condition that's not ideal and you're going to have games that will be played at very high temperatures, so I think it's going to be very difficult for all the teams.
"I do not predict great football because, with high temperatures, the pace will not be very high, and people have become used to watching high-paced games.... It will be difficult for everyone with surprises possible in every match."
Sarri was criticised after Juventus lost the Coppa Italia final to his old club Napoli last week but isn't concerned as his team sit top of the league.
He said: "In life, there are facts and there are opinions.
"The media said that Juventus as a team were finished, but the standings show we are top of the table. We can use the criticism as motivation to get back into shape physically and mentally."