Coach Steve Spagnuolo on Friday said that if Bulger dresses for the game, it will be as the third quarterback. Technically, Bulger is listed as questionable.
The winless Rams play at NFC West rival San Francisco (2-1). St. Louis will try to end a 13-game losing streak, the longest in the NFL.
Bulger hurt his right shoulder in the first quarter of last week's 36-17 loss to Green Bay. Boller came in and threw two touchdown passes, but the comeback fizzled in the second half.
Boller is in his first year in St. Louis. He had 42 career starts for Baltimore, but Sunday will be his first start since Dec. 16, 2007.