The Portuguese forward was substituted in back-to-back encounters before the international break, with the club claiming it was down to fitness.
Ronaldo then went on two complete two full matches for Portugal and bagged four goals during the FIFA break, making the injury claims doubtful.
Cassano feels the two substitutions against Lokomotiv Moscow and AC Milan has created a problem between the pair.
"I think Ronaldo has a big problem with Sarri," told Tiki Taka.
"Everything started with two consecutive substitutions, which will irritate any player.
"In my opinion, there's a problem between them and Juventus are good at keeping it down."
The five-time Ballon d'Or winner has registered six goals in 11 Serie A appearances this season.