It didn't matter to the Mavs, though, who defeated the Thunder 103-93 for their 17th victory in 18 games.
Nowitzki scored 13 points before leaving with 9:10 left in the second quarter. The Mavericks said it was a sore right knee.
Despite leaving the game, the injury reportedly isn't serious.
X-rays after the game were negative and an MRI is set for 9:00 a.m. CT Tuesday.
Mark Followill, the television play-by-play announcer for Mavericks broadcasts on FoxSports Southwest and TXA-21 said after the game in a tweet that he was assured by the Mavs assistant coach, Dwane Casey, that the injury shouldn't get Mavs fans worried.
In a sign of the Dallas-Mavericks-deep-bench-times, Shawn Marion chipped in 20 points and nine rebounds in replacing Nowitzki, a popular choice for early-season MVP considerations.
Dallas improved to 11-1 on the road this season, which is currently the best road mark in the NBA. The Mavs are also 24-5 overall, good enough for second place in the daunting Western Conference.
The Mavs face the Raptors on Tuesday. Nowtizki will be questionable for the game.
Information from the Associated Press was used in this report.