The Spaniard was booked for a tackle on Kasper Dolberg in Real's 2-1 first-leg victory in Amsterdam on February 13, two minutes after Marco Asensio scored the winning goal.
Three yellow cards bring a one-match suspension but Ramos may have seen sitting out the second match against Ajax as preferable to potentially missing a match later in the competition.
UEFA said that Ramos' case would be dealt with on Thursday.
European football's governing body set a precedent last season when Real full-back Dani Carvajal was suspended for two games after initially earning a booking in the group stage.
The 32-year-old Ramos said the day after the match that he did not "force the card".
But straight after the game, he had appeared to admit that he got himself booked on purpose.
"The truth is that given the result I'd be lying if I said it wasn't something I had in mind," Ramos had said.
"It is not to underestimate the opponent but sometimes there is a time to make decisions and I have done so."
Real resume their bid for a fourth consecutive Champions League title in the return leg against Ajax at the Santiago Bernabeu on March 5.