Tim Duncan will miss his fourth straight game due to knee soreness on Monday.

Gregg Popovich said Wednesday that Duncan underwent an MRI on his right knee, and that he was in the process of rehabilitating it. 

Duncan is averaging just 8.9 points and 7.5 rebounds per game this season but he remains a key part of the Spurs' defense.

The Spurs don't have a publicly available timeline for when Duncan will return.