In fact we have to realize that Ronaldo has been beaten by age productivity has dropped drastically and maybe in 3 years he will retire
For now nothing can stop Manchester City in the Premier League and Arsenal are the most obvious competitors yes at least before the middle of the season.
But that doesn't justify him been benched throughout the game. Somehow Manchester united managed to get three goals from that game and with Ronaldo on the pitch, there's every chance you can get a goal or two from him and let say he played and got a goal then the pressure would had been on Manchester city which might result to them playing under pressure and Manchester united would had then taken advantage of that situation to counter play them as they usually do to opponent that seems to outplay them in open play and a perfect example was the arsenal game.
Ronaldo should be used as super sub, there was a reason Manchester city were attacking united every chance they had, it was because they knew there was no one in the Manchester united team that could cause their defense some problem. Back in the days of Sir Alex Ferguson, when Ronaldo was younger, we saw Sir Alex bringing in supper sub of older players later in the game and that's how a manager utilized their older players that has quality in them. Same should be done about Ronaldo instead of disrespecting him always.