Team spokeswoman Sarah Melton said an MRI revealed the sprain and Nowitzki is day to day.
"We'll see how it goes," Mavericks coach Rick Carlisle said before the game. "We're not sure what the timetable will be."
Nowitzki was injured in the second quarter of Monday night's 103-93 victory at Oklahoma City. He came up limping after falling awkwardly when he was fouled. He had 13 points when he left the game with 9:10 remaining in the first half.
The 7-foot German is seventh in the NBA in scoring at 24.1 points per game.
Shawn Marion started at power forward Tuesday.
Carlisle was back on the bench after missing Monday night's game following knee surgery. Assistant coach Dwane Casey handled the team in Carlisle's absence.