Juve return to Serie A action against struggling Milan, who sit 12th, with new coach Stefano Pioli having managed just one win from four matches in charge.
Asked for an update on Ronaldo's fitness on Saturday, Sarri revealed he is still uncertain if the former Real Madrid star - who has six goals in all competitions this season - will be able to play.
"How is Ronaldo? I still don't know, because yesterday he was with the physiotherapists and we will see today and tomorrow in the last training session," Sarri told a news conference.
"Let's see what he can do. He feels a little pain in his knee but it is nothing serious."
Ronaldo looked set to open the scoring for Juve in midweek when his free-kick squirmed out of Lokomotiv goalkeeper Guilherme's grasp, only for Aaron Ramsey to ensure the ball went over the line.
It brought up Juve's 300th Champions League goal, as well as marking a second strike of the season for Ramsey, who has been restricted to eight appearances.
Sarri, however, is looking forward to having what he sees as a crucial player more readily available.
"He has important technical qualities and can do both the roles I ask," Sarri said of Ramsey.
"He can be a modern midfielder. He can be an offensive midfielder, as an attacking midfielder he can also give cover."
Sarri has previously aired his frustrations with some profligate finishing from Juve this term, but the ex-Chelsea coach has been pleased with his side's ability to push for late winners.
"The fact is that the team is still unable to close out games," Sarri said. "The strength, however, is that until the end we go to look for the victory and often we succeed.
"Milan is a good team, which did not make a good start to the season. In football there are cycles; rebuilding to get back to a very high level is not easy. Milan have many young players and they are laying the foundations for an important future."