Report: Willis has hand fracture
On Thursday, Willis was wearing a black cast around the base of his right hand and wrist and was not participating in practice.
The injury is not considered serious, according ESPN's sources.
Willis was injured in one-on-one blocking drills during Wednesday's practice. He was among more than a dozen players sitting out Thursday, including fellow All-Pro middle linebacker NaVorro Bowman, who has been dealing with a groin injury.
Coach Jim Harbaugh was scheduled to address reporters Thursday afternoon.
Willis has made the Pro Bowl in each of his first six seasons and has been an Associated Press first-team All-Pro five times. He had 88 tackles, two forced fumbles and two interceptions to anchor San Francisco's stout defense last season.
The Associated Press contributed to this report.