The team officially listed the ailment as a right knee effusion. Coach Rick Carlisle said the team wasn't certain when or how Nowitzki, 37, injured the knee or if he would miss multiple games.
Nowitzki, 37, sat out three previous games this season, but none due to injury. Right knee surgery cost him the first third of the 2012-13 season.
Forward Charlie Villanueva started Wednesday in place of Nowitzki, who is averaging 17.8 points and 6.9 rebounds per game this season.