Earlier in the day, Rams coach Sean McVay said that Henderson would not practice due to an illness and that further tests would be run to determine whether his sickness was COVID-related.
McVay has said this season that to his knowledge every player with the exception of one has received the COVID-19 vaccine and that the lone player who was unvaccinated has since gained COVID-19 antibodies.
Henderson is among those who are vaccinated, so he will need two negative tests within 24 hours to be eligible to play.
The Rams (8-4) play the division-leading Arizona Cardinals (10-2) at State Farm Stadium on Monday Night Football.
In the likely case that Henderson will not be available, Sony Michel will start for a second consecutive game.
Michel is coming off a season-best performance after he rushed for 121 yards and a touchdown on 24 carries in a 37-7 victory over the Jacksonville Jaguars on Sunday.
Michel started in Henderson's place as the third-year pro nursed a thigh injury and did not play.
Barring a roster move, Mekhi Sargent is now the only backup running back behind Michel. Sargent is an undrafted free agent from Iowa.
The Rams also are dealing with uncertainty on the offensive line as they prepare for Monday's game.
Shelton stepped in on the first series against the Jaguars when Allen suffered the knee injury.
"You'd love to be able to have Brian," McVay said. "But if anything, Coleman played the whole game last week with the exception of the opening snap. I thought he did an outstanding job."