Maybe it's just circumstances. He was coming off a stomach virus when the Mavs lost to the 76ers. He got in early foul trouble in the rout of the Knicks and watched the fourth quarter from the bench with the rest of the regulars.
Howard definitely hasn't helped his trade value this month. He's averaging only 10.5 points and 3.1 rebounds while shooting 34.6 percent from the floor in January.
Of course, his contract status ($10.89 million salary with a team option at $11.8 million for next season) has more to do with his trade value than his talent in today's NBA.