An A-ray this morning revealed that the South African second row has an undisplaced fracture of his radius. He is expected to be out for at least eight weeks.
It is cruel luck on Muller who only returned for the province from a hand injury last weekend, lining out in the RaboDirect PRO12 win over the Scarlets.
His latest lay-off will see him miss the return fixture with the Saints at Ravenhill, the Christmas time league derbies and Ulster's closing pool games in Europe next month against Glasgow Warriors and Castres Olympique.
