Parker, coming off three consecutive 1,000-yard seasons, injured his medial collateral ligament Sept. 21 in Philadelphia. He ran for more than 100 yards in each of the Steelers' first two games.
Parker was back in practice Monday and looked ready to return, only to aggravate the injury while working out on his own Tuesday, according to coach Mike Tomlin.
"We're still hopeful toward Sunday," Tomlin said Thursday. "But maybe not as hopeful as we were earlier in the week."
Mewelde Moore, normally the third-down back, would start Sunday if Parker can't play. Moore ran for 99 yards against Jacksonville on Oct. 5, his first NFL start since he played for Minnesota in 2005.
Left tackle Marvel Smith (back spasms) also didn't practice, and nose tackle Casey Hampton (groin) did only limited work. Their status for Sunday remains uncertain. Hampton hasn't played since the 15-6 loss in Philadelphia.