It's the same as for most two-pitch hurlers. If batters aren't forced to respect a breaking pitch that will just cross the strike zone, then they will simply sit on the heater. If he's leaving those fastballs in the middle of the zone, so much the worse. Command has to be there every outing, or he's just another arm in somebody's bullpen. Most of baseball is complicated at the major league level, but I think this is one instance where it isn't.