The two-time Pro Bowl player broke the news on his Twitter feed after undergoing an MRI exam Monday, saying he'd already had two hours of rehab treatment one day after the Rams' 30-16 victory over the Washington Redskins.
"Steven's a warrior, and you almost have to pull him back," Spagnuolo said. "What you don't want to do is get him out there too soon and set him back. We're all happy it wasn't anything worse, and now it's just a matter of treatments and making sure we don't make it any worse."
The Rams' most serious injury from the Redskins game was to wide receiver Dominique Curry, who will undergo surgery for torn ligaments on his right knee and will be placed on injured reserve. Curry was one of the team's top special teams players, and he blocked a punt Sunday before feeling something pop in the knee while making a cut.
"He was really coming on," Spagnuolo said. "He had done a lot of good things."
Spagnuolo said safety Oshiomoghe Atogwe, the defensive captain, aggravated a thigh injury and might be a question mark. Atogwe missed the second half.
Jackson scored on a 42-yard run for the game's first touchdown and finished with 58 yards on 10 carries.
He was hurt during the second quarter of Sunday's win, which snapped the Rams' 14-game home losing streak. He was gang-tackled on a carry, and replays showed a Washington player yanking one of Jackson's legs.
"He's sore, and he would be because you can see on the film," Spagnuolo said.
Backups Kenneth Darby and Keith Toston filled the void. Darby scored on a 12-yard run for his first career touchdown, and the pair combined for 71 yards on 25 carries. Spagnuolo said Toston played through a minor shoulder injury.
"Real solid," Spagnuolo said. "You're talking about guys who hadn't gotten a lot of reps in the course of the week."
The Rams are likely to shop for a more experienced backup, but Spagnuolo said he'd be happy going with that duo again if Jackson's not ready.
"Based on yesterday, yeah, how could I not be?" Spagnuolo said.
Copyright 2010 by The Associated Press