Ronaldo's suspension ahead of Man Utd clash; what the rule states
According to the UEFA regulations, a sending off means the player will be suspended for the upcoming game in UEFA competitions. However, should UEFA deem the incident as a serious offense, then the suspension could be extended.
"As a rule, a player who is sent off the field of play is suspended for the next match in a UEFA club competition (i.e. UEFA Champions League, UEFA Europa League or UEFA Super Cup). In case of serious offences, the UEFA Control, Ethics and Disciplinary Body is entitled to augment this punishment, including by extending it to other competitions," article 49 states.
In other words, given that the sending off was very harsh, Ronaldo almost certainly won't miss the game against his former side in October.