Ulster captain Rory Best could be ruled out for up to six weeks after picking up a hamstring injury on Thursday in the province's latest injury setback.
Best could be a doubt for Ireland's autumn international series starting against South Africa on 11 November.
The Ireland skipper is on crutches and awaiting results of a scan after suffering the injury in training.
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 against the Cheetahs and Benetton Treviso.
The trio were also not selected for Friday night's home clash against last year's champions Scarlets at Kingspan Stadium.
Deysel was named at number 8 before picking up a "minor eye trauma" in training on Thursday.
Ulster are again without star duo Coetzee and Piutau who didn't feature in Saturday's lacklustre win over Benetton.
Ulster confirmed that both players have been carrying knocks but both are expected to be available for next weekend's game against the Dragons.
Gilroy has been ruled out for three months with a stress fracture in his lower back.