Sims had the appendectomy Sunday night.
Indianapolis signed the 26-year-old free agent to a one-year contract last week. Sims had more than 100 tackles in each of his first three seasons with Detroit, playing in the same defensive system the Colts use. But after a coaching change, his production dropped in 2009 and the Lions wound up trading their first-round pick from 2006 to Philadelphia.
Sims is hoping to revive his career with the Colts by competing for the vacant starting job at weak side linebacker. That position opened up when last year's starter, Clint Session, signed with Jacksonville.
Caldwell did not specify when Sims could return.