Nowitzki, Dallas' All-Star power forward, already has missed two games this season due to an injured left ankle and did not return after hurting it again late in the second quarter.
The Mavericks managed to match Portland until Nash took over in the fourth. He hit a running jumper and three-pointer to keep Dallas ahead and made nine of 12 free throws, including seven of in the final 40 seconds, to hold off a late rally.
Michael Finley scored 20 points despite hitting only seven of 26 shots and Tim Hardaway added 12 off the bench for the Maverickss, who avoided their first four-game losing streak since the 1999-2000 season.
Rasheed Wallace and Ruben Patterson scored 16 ponts apiece for the Blazers, who have lost three of their last four games.