Sanders said his back didn't seem to be healing and he hadn't had this much pain from any bone bruises in the past.
A bone scan conducted after Sanders fell hard on Feb. 5 at Denver did not show any fracture.
"We're probably going to make arrangements for him to go see somebody," Bucks coach Jim Boylan said before Milwaukee hosted Washington.
Boylan said Sanders still is listed as day-to-day.
"We'll see where we stand tomorrow and Wednesday, but for right now, tonight, he's out," Boylan said.
A strong defender who has blocked at least one shot in 33 consecutive games, Sanders is averaging 8.5 points, 8.5 rebounds and 3.1 blocks.