Foster suffered a back injury in the first series against the Indianapolis Colts in last Sunday night's game.
Phillips said Foster, who did not practice all week, has seen multiple specialists about the injury. According to a source, Foster visited with three specialists this week, including one who is out of state.
Foster went into the Colts game questionable with a hamstring injury that prevented him from finishing the Texans' Oct. 20 loss to the Kansas City Chiefs.
Foster has dealt with injuries to his back, calf and hamstring since the start of training camp. He has rushed for 542 yards and one touchdown in eight games.