The 34-year-old Portuguese international was rested for Saturday's Serie A game against Atalanta which Juventus won 2-1 thanks to a Gonzalo Higuain double to stay top of the Italian league, AFP reported.
"Cristiano trained with his teammates yesterday (Sunday)," said Sarri.
"It seems that his condition is improving but we still have to evaluate between today and tomorrow.
"The positive thing he was able to play throughout the training session with his teammates."
Ronaldo was substituted in Juventus's previous two games including their Champions League game at Lokomotiv Moscow with a slight knee problem.
The five-time Ballon d'Or winner hit back with four goals to help Portugal qualify for Euro 2020.
"My relationship with Cristiano is good," said Sarri, "if a player who has won so much is angry about being substituted, it means he still has a lot of motivation, and this is something positive."
Juventus is already through to the last 16 and want to finish top of Group D with second-placed Atletico needing a win to guarantee their place.
"Atletico is a difficult side to play against, we have to think we want to qualify being the leader of the group that is our target for tomorrow," said Sarri.
Sarri conceded he was spoilt for choice up front between Ronaldo, Paulo Dybala and Higuain.
"We have three very strong strikers for just two roles, I think it is an ideal situation, nobody will be able to play 65 matches, including with the national team," continued the Juventus coach.
"Playing Dybala, Higuain and Cristiano all together is difficult because of their characteristics, but all three know how to play really well and are comfortable with each other."