After Jackson watched practice in sweats, coach Jim Haslett figured the Rams would be without their feature back for the third time in four weeks.
"I doubt if we'll have him, to be honest," Haslett said. "But we'll see. You never know."
If Jackson can't go, Antonio Pittman would get his fourth straight start on Sunday at San Francisco. Pittman has been limited by a right hamstring pull but said he's feeling much better.
Jackson was hurt while landing awkwardly in the fourth quarter of a victory over the Dallas Cowboys on Oct. 19. He has 525 yards rushing, a 4.1-yard average and four touchdowns and is tied for second on the team with 25 catches.
"I really didn't think I would miss a game, let alone two or three," Jackson said. "For it to linger like this, it really begins to make you wonder what's really going on."
The Rams (2-7) haven't changed their diagnosis of a strained thigh. Jackson said doctors have told him two MRI exams have revealed mostly swelling and inflammation.
Jackson thought he had turned the corner after practice on Wednesday, when he took about half the snaps. That was his first full day of practice since the injury.
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"You know what? I really thought I was pretty much there," Jackson said.
Still, Haslett said Jackson was "probably questionable to doubtful" for Sunday even before the soreness kept him out of Thursday's practice.
"We thought he ran well on Monday and he conditioned on Tuesday, and he looked bad yesterday," Haslett said. "He's going to have to run before he can play."
In retrospect, Jackson wonders whether it would have been better to let the injury heal for a couple games without making an effort each week to return.
"I don't blame anyone, and I want to get out there more than anybody," Jackson said. "Maybe I just have to sit myself down and kind of allow it to heal itself."
Jackson second-guessed his decision to play against the Cardinals two weeks ago. He didn't start because the first play required a cutback he wasn't comfortable making, and he finished with 17 yards on seven carries and dropped a sure touchdown pass.
"If I'm not 100 percent, I think we saw in Arizona that I'm just a wounded duck out there," Jackson said.
Pittman's numbers haven't been pretty the last two weeks, with 23 carries for 40 yards while hindered by his own hamstring. He's felt much better in practice this week.
"I'm aware of what he means to this team, and I'll try to go out there and give it my all," Pittman said. "That's all I can do, leave it all on the field."