The 11-time All-Star played 40 minutes a night earlier in the Spurs' 105-85 home win against the Los Angeles Lakers. San Antonio has another back-to-back set Friday at Charlotte and Saturday at Memphis during a four-game trip.
Duncan was on the Spurs' active list but wore a dress shirt and jeans with a grey sport coat.
He was replaced in the starting lineup by rookie DeJuan Blair.