The striker appeared to knock Jeison Murillo to the ground off the ball and was shown a straight red card by the referee on the 28-minute mark.
The Portugal forward placed his hand on the Valencia defender's head and referee Felix Brych decided to dismiss Ronaldo after discussing the incident with his assistant behind the goal.
Ronaldo looked baffled following the red card and continued to profess his innocence.
The Portugal star was seen crying as he left the pitch, still shaking his head.
It was his first red card in 154 Champions League games, and the decision means he could miss a return to Old Trafford when Juventus plays Manchester United next month on 23 October.
The 33-year-old Ronaldo joined Juventus from Real Madrid in the off-season and scored his first two goals for the Italian side this past weekend in a 2-1 win over Sassuolo in Serie A.
Ronaldo is the Champions League all-time leading scorer with 120 goals.
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