Ronaldo featured in both of Portugal's 2020 European Championship qualifiers during the international break and showed no signs any injury.
However, the Portuguese forward was left out of the squad that defeated Atalanta on Saturday afternoon, with Sarri revealing after the game that the forward has had an issue with his knee.
"I hope he'll be back for the Champions League on Tuesday. He has this little problem that is above all affecting him in training, as he's struggling to train consistently," Sarri said, as per Football Italia.
"A lot depends on how the lad feels, the fatigue and the need he has at any moment to rest. He felt, and I fully agreed, it was important right now to find a way of training that wouldn't put extra pressure on the knee.
"It's a solid knee, there are no lesions, but constantly having that pain does take it out of you. I was happy to rest him, as I felt the same way, that it was best to take this time to first of all rest and secondly analyse training methods to see how he could train differently."
In Ronaldo's absence, Gonzalo Higuain bagged a goal and an assist as the Old Lady defeated Atalanta 3-1 on Saturday afternoon.