Alexander, 26, had started the first four games. He was replaced by rookie John Johnson, who intercepted a pass and returned it 69 yards.
Coach Sean McVay said after the 16-10 loss to the Seahawks that Alexander was not inactive because of a disciplinary issue.
Alexander was selected in the fourth round of the 2014 draft from Utah State. He started 14 games at free safety in 2016, but moved to strong safety this season as Lamarcus Joyner switched from linebacker to free safety.
The Rams now have three safeties on the roster: Joyner, Johnson, Cody Davis. Marqui Christian and Blake Countess, who are listed as defensive backs, can play safety.
Thomas rejoined the team last week after serving a four-game suspension for violating the NFL’s policy on performance-enhancing substances. He was on the exempt list and was not active for the game against the Seahawks.
Follow Gary Klein on Twitter @LATimesklein