What do you mean by "Kepa was in the right"? If Kepa believed he should not have been pulled for any reason (that he thought he could still go on and perform etc.) he should at the very least come over to the sideline to explain/present his case to Sarri, instead of publicly defying and insulting Sarri.
Blatant insubordination and very unprofessional.
Sarri said Kepa was signalling he has a cramp for a few minutes before that. And if you were watching the game you can definitely say he was cramping up, he was sat in goal holding toes of both feet at one point. That said, he was saying later he's OK to play and that sub is not needed.
Basically it comes down to a) this being a plan for penalties before the game - which I don't think it was, Caballero doesn't have some remarkable record, and b) Sarri being a bit hot in his head at the time and trying to do his job arranging the best possible plan for pens. Shit is tense in London in the past few months. Sarri made the sub, was sitting down and got told the player doesn't want to come off. On an instinct it's basically a player telling you to fuck off. Which now doesn't seem to be the case.
And he couldn't have come to the touchline. He knew the board would have gone up as soon as he made the move and he'd be forced to come off just on the basis of not getting booked for time-wasting.
It looked horrible on TV, but got all jumbled up and emotive. Captains should have communicated it better really.