Jackson was hurt on a 2-yard carry to the Washington 40 with 4:02 to go in the second quarter Sunday, getting his body twisted by a number of tacklers. The ball came loose at the end of the play and was initially ruled a fumble, but the Rams won an appeal.
St. Louis coach Steve Spagnuolo was hopeful that Jackson, who'll undergo an MRI exam Monday, could play next week at home against the Seahawks.
"All the signs point to that it's not a tear," Jackson said. "I'm just praying and hoping it's not going to be a serious injury."
Jackson was down for several minutes while the play was being reviewed. He received treatment on the sideline before walking to the locker room.
Jackson scored on a 42-yard run in the first quarter for his first touchdown of the season in the first quarter. He had 58 yards on 10 carries.
Information from The Associated Press was used in this report.