Coach Pete Carroll said earlier this week that Thomas had a good week of practice after returning to the team Wednesday from his holdout. At the time, he wouldn't say whether Thomas -- the final member of the "Legion of Boom" -- would play.
"He was solid all week long," Carroll said. "He's fired up and studying and working hard at it, and he had a good week. We'll let you know how that's going to turn out here tomorrow."
Thomas, 29, held out all offseason while seeking a contract extension or trade to a team that would give him one. He implied that he took a discount in the past to keep the Seahawks' storied defense together.
In a post on Instagram on Wednesday, Thomas wrote that he wasn't going to let his teammates, the city or fans down and that "the disrespect has been well noted and will not be forgotten.
"Father Time may have an undefeated record, but best believe I plan on taking him into triple overtime when it comes to my career," he wrote.
Thomas' starting so soon after returning from his holdout wouldn't be at all unprecedented. Fellow safety Kam Chancellor returned on Wednesday of Week 3 during the 2015 season after holding out during training camp. He started and played 37 of 47 defensive snaps that Sunday in a home win over the Chicago Bears.
The Seahawks on Saturday also placed cornerback Dontae Johnson on injured reserve. The projected starter on the right side, Johnson popped up on the injury report this week with what Carroll called a groin strain.
Johnson will be eligible to return off IR in two months.
Tre Flowers, a rookie fifth-round pick, is one option to start at right cornerback. Seattle also has veteran Neiko Thorpe, who has mainly been a special-teams player in his two seasons with the Seahawks and missed a good chunk of training camp with a wrist injury.
Flowers has impressed the team in his conversion from college safety to cornerback, with coach Pete Carroll saying Friday that Flowers has "done everything we've asked him to do."
ESPN's Brady Henderson contributed to this report.