With the ball on the left wing, Ronaldo tried to make a run into the penalty area and was blocked off by Jeison Murillo.
There was a small push from Ronaldo which resulted in the Valencia defender falling to the floor but that was not violent conduct.
The referee was unsighted for the incident, however his assistant referee spotted the incident and alerted the referee,
The former referee and current 'BT Sport' pundit said: "Ronaldo was then clearly annoyed by Murillo’s play-acting and placed a hand on the defender’s head.
"The contact looked very slight and I cannot see how the additional assistant, standing some 20 yards away, could deem it violent conduct.
September 19, 2018
"If I had been officiating this game, I would have given Valencia a free-kick and shown a yellow card to each player. Ronaldo will serve a one-match ban but anything more would be harsh."
Ronaldo's former Manchester United team-mate, Rio Ferdinand, believes that he should appeal the red card - with Ronaldo potentially due to miss both Manchester United Champions League fixtures.