The Reds took to the field at Selhurst Park with four changes from the side that started against Real Madrid in midweek, with Keita one of those coming in.
However, after picking up a yellow card in the 23rd minute and proceeding to commit three more fouls, the Guinean was hooked at half-time, ending a woeful display.
Alexander-Arnold was similarly poor, with the 24-year-old a jaded figure at right-back, and he was the next to be brought off along with Diogo Jota.
It was just clear, Klopp told reporters of the decision to take Alexander-Arnold off.
Trent plays all the games for us, then we have to make [decisions] from time to time.
We thought before the game already if it was one too much for him or whatever, but then he started. Thats it.
The reason was purely because of that. Nothing else.
On Keita, he explained: Naby had an early yellow card, then two situations after that which took him obviously probably out of the game.
He realised from now on no contact with anybody anymore. So that was the reason.
In his post-match press conference, Klopp also confirmed that Joe Gomez missed the game with a muscle injury, and discussed Darwin Nunezs shoulder problem.
Darwin against Newcastle, when he fouled Trippier but it was obviously a big joke, [injured] the shoulder, he said.
It was OK with an injection against Madrid, because theres no structural damage its very painful.
But he got another knock there and the day after the Madrid game he couldnt move the arm at all.
We have to see how long that will take, but that was the reason for him [not playing].