The 4-time Pro Bowler will undergo an MRI to confirm the injury.
According to ESPN.com's Nick Wagoner, the injury is to Long's right knee, the same one Long injured in Week 16 last season. Long tore both his ACL and MCL in that game, and underwent surgery in January to repair the ACL.
Long was injured late in the third quarter Sunday while quarterback Austin Davis was being sacked for a 5-yard loss.
In addition, Rams wide receiver Brian Quick suffered a dislocated shoulder and torn rotator cuff in Sunday's game and now might miss the rest of the season, a league source told Schefter.