The 31-year-old former Ulster captain lasted just a quarter of an hour of Ireland's heartbreaking 24-22 defeat to New Zealand in Dublin, with his injury ironically coming when he scored his country's second try in a blistering start at the Aviva Stadium.
No official diagnosis has yet been made but an Irish Rugby Football Union spokesmen stated that the British & Irish Lions summer tourist was struggling with a 'serious lower arm injury'.
The news is desperately disappointing for Ulster as they get set to return to Heineken Cup action in less than two weeks' time. The 1999 European Champions have started their latest quest for continental glory in fine form to top Pool 5 after two rounds of action, but losing Best for the December double header with Benetton Treviso and the January clashes with Montpellier and Leicester Tigers is far from ideal.
Best was a key figure in both of his side's October victories in Europe as Ulster began with a bang by beating Leicester 22-16 at Ravenhill in Round 1 before shocking Montpellier at the Stade Yves du Manoir. Best and co became the first side to topple Montpellier at home this term and, in doing so, took control of the group with four games remaining.
He seems certain to be forced out of the games against Treviso in Belfast on Saturday December 7 and at the Stadio di Monigo exactly a week later but arguably of greater concern to head coach Mark Anscombe will be his potential unavailability for the home tie with Montpellier, who are just three points off the head of the table in the Top 14, and the mouth-watering trip to Welford Road when the pool stages come to a close on the weekend of January 17/18/19.
Best was in line to hit a half century of Heineken Cup appearances prior his arm injury, with Ireland's first-choice No2 just five games shy of that mark. He has started 44 times and come on once as a replacement since his European debut back in the 2004/05 campaign and hasn't missed a Heineken Cup tie since December 2010.
He started all seven games as Ulster reached the quarter-finals last time out and had again been an ever-present when they made it all the way to the Heineken Cup Final the previous year. The last time Ulster named a Heineken Cup starting side without Best in it was when they won at Bath in the pool stages just under three years ago.