Ulster and Ireland captain Rory Best has been ruled out for four to six weeks after a scan revealed he had suffered a hamstring tear in training.
Best has been on crutches since sustaining the injury on Thursday.
The hooker is likely to miss Ulster's first two Champions Cup pool games, at home to Wasps on 13 October, and away to La Rochelle nine days later.
Best may also be a doubt for Ireland's autumn international series, starting against South Africa on 11 November.
Leinster full-back Rob Kearney is also a concern for those November games against the Springboks, Fiji and Argentina after being ruled out for up to eight weeks with a hamstring injury.
Flanker Dan Leavy, who has impressed in his four Ireland caps to date, will also miss up to eight weeks after undergoing surgery on an ankle problem.
Ulster are already without Marcell Coetzee, Charles Piutau, Jean Deysel and Craig Gilroy through injury.
Best captained the British and Irish Lions' midweek side in New Zealand for the second successive tour and has not played since the tourists' draw against the Hurricanes in Wellington on 27 June.
Along with fellow Lions Iain Henderson and Jared Payne, the 35-year-old sat out Ulster's opening Pro14 wins over the Cheetahs, Benetton Treviso and Scarlets.
Coetzee and Piutau are expected to be available for this weekend's game against the Dragons after missing the Irish province's last two fixtures.
Gilroy has been ruled out for three months with a stress fracture in his lower back.
Deysel was named at number eight for Friday's hard-fought 27-20 victory over the Scarlets, but was forced to pull out of the game after picking up a "minor eye trauma" in training on Thursday.