Nash received seven stitches but returned to start the second half.
The two players made contact when Watson drove to the basket with just more than four minutes left in the first quarter. A Suns spokeswoman said a tooth went through Nash's upper lip. Nash received four stitches on his upper lip and three on the inside of his mouth.
No foul was called on the play.
Nash, a two-time NBA MVP, was averaging 19.2 points per game and leading the league with 11.3 assists a game.