Anderson suffered a groin injury during the pre-season game against the Houston Texans Friday night.
"He's got two muscles in that groin area and he tore both of them off the bone," Coach Mike Shanahan said on the team's Web site. "Some people have had surgery. The specialist recommends just to let it heal. Right now I'm not sure if the decision's final, but I don't think surgery will be needed."
Anderson was told the extent of the injury after getting results of a second MRI Monday. He likely will be placed on injured reserve later this week.
The 30-year-old Anderson rushed for 257 yards during an injury-plagued 2003 season.
In his career, he has 2,821 yards and 24 touchdowns in 59 games.