Rookie Zac Stacy has been a big part of the St Louis Rams' revitalized rushing attack.
But whether Stacy, who has rushed 141 times for 624 yards and four touchdowns, will get a chance to pound away at the San Francisco 49ers on Sunday is up in the air.
Stacy was one of three Rams players who left Sunday's 42-21 win over the Chicago Bears with concussion-like symptoms.
Cornerback Trumaine Johnson and guard Chris Williams also were sidelined.
Rams coach Jeff Fisher said when a player has concussion-like symptoms, there is a process that must be followed before the player gets back on the field.
None of the three Rams who left against Chicago made it back into the game. It is not known yet whether they will be able to play against San Francisco.