DALLAS — When Jamal Murray went to the bench with 6:38 remaining in the third quarter on Sunday, the Nuggets were losing to the Mavericks by 12 points. They were losing his minutes by 29. Individually, Murray had five points, no assists and a turnover on 2-of-7 shooting. His defense wasn’t any better. As he took a breather, an important game spiraled further out of control.