Nash was lunging for a loose ball midway through the first period when his neck came in contact with the hip of Dallas forward Tyson Chandler. Nash told trainers he felt numbness in his neck and shoulder and he did not return to the court.
With their playmaking guard on the sidelines, the Suns scored just 17 points in each of the middle two periods and trailed by 21 heading into the fourth quarter.
The Mavericks won for the 14th time in 15 games.
Caron Butler added 17 points for Dallas and Shawn Marion had 15. Chandler collected 13 rebounds.
Hakim Warrick led Phoenix with 18 points. The Suns shot just 39 percent from the field.