Source: Royce Young, CBS Sports 
Gregg Popovich told reporters at shootaround Duncan will sit Friday’s game against the Lakers because of a chest contusion he suffered Wednesday against the Grizzlies.
Duncan is listed as day-to-day, though Popovich said he was unsure of when he’d return.
This is always the concern with the Spurs every season as their core ages, that injuries will pile up and eventually the Spurs won’t be able to keep their best players on the floor. They’ve managed it for a number of years already, but each year, it becomes more and more challenging.