Ronaldo said there needs to be a change in attitude and mentality among the players if United are to have any chance of challenging for the major trophies again.
Rangnick said: “If it's necessary to speak in a very direct manner to the boys, it helps a lot.
“If Cristiano, for example, or any of the other players address that to the players directly on the pitch or in the dressing-room, they are more than welcome to do that.
“I will do that myself as a coach and my coaching staff will.
“If one of our players in the game doesn't give his very best and put all the effort into it, then I will address that, you can be assured of that.
“So far I haven't seen that but, with some players, they start well. We can speak about Marcus Rashford, for example.
“I think in the first 20 minutes against Aston Villa he was good, he was on fire, he was emotionally involved.
“Then, after the first 30 minutes, he got worse and he didn't play on the same kind of energetic level he did in the first half an hour.
“But I don't see any of the players who don't care about the club or who don't want to perform to the best and highest possible level.
“If things don't go the right way, you have to behave in a certain manner, and it's about how you deal with those situations."