Real Madrid beat relegated Real Oviedo 2-0, with Gonzalo García scoring before half-time and Jude Bellingham adding a second in the 79th minute assisted by Mbappé, who had come on in the 68th minute to a wall of whistles from his own supporters.
"I did not tell him any such thing. He probably did not understand me well. I don't know what to tell you. I have not said he is the fourth striker."
When pressed further, Arbeloa made his position on the matter permanent.
"While I am in this chair, I decide who plays, whatever their name. If they are happy with that, fine. If not, they can wait for whoever comes next."
He acknowledged understanding the frustration of any player left out of a starting eleven.
"I can perfectly understand that Kylian is not happy today about not playing. But the decision was based on circumstances. Four days ago he could not even make the bench. It was not a final today, it was not a life-or-death match. Gonzalo deserved the reward of starting. It was the most logical, natural decision."
On whether any distance had opened up between him and Mbappé.
"No. A player who could not be on the bench four days ago should not start. This is not the first time it has happened to me — not only with Kylian, with more players than you can imagine. I am very calm and I have a very clear conscience. It does not matter to me that they do not agree with my decisions."
On the booing directed at Mbappé during his substitute appearance, and separately at Vinícius Júnior.
"I always ask for support for the players. They are pillars of this club, the club believes in them, and the applause will return."
He added, unprompted, that he did not consider any of his players ungrateful.
"I do not expect from my players what I gave for them. I know what we are as players. Right now I am in this chair, but one never stops being a footballer. I know how they feel, what they think. Players of this level, with ambition and a great ego — I am capable of understanding it all. I would not change anything I have said or done, including defending all of them."
Two games remain for Real Madrid — Sevilla away and then a final home fixture — as the club prepares to conclude a second consecutive trophyless season.