Williams left Sunday's 31-28 loss to the Denver Broncos with back spasms, though initial X-rays were negative, offering hope the problem would not impact his participation against the Jags.
The 28-year-old has not practiced all week, although head coach Brandon Staley said Williams needed time off, having had chronic back issues for years.
"He's progressing, getting treatment," Staley told reporters. "Just trying to get him as much rest as possible because of the short time frame of the week.
"We're at the point of the season where the reps aren't what matters – the rest is – in his case. Just trying to get him as much rest up until game time."
Under-fire Staley was criticised for not resting key players against the Broncos, given they were locked into the fifth seed in the AFC playoffs ahead of the game.
"I stand behind what we did in that football game," Staley added on Thursday. "I didn't want anybody to get hurt in that game regardless of their status."
Williams has 63 receptions for a team-high 895 yards with four touchdowns this season.