Rory Best looks set to be ruled out for up to six weeks after picking up an injury.
Ireland captain Best sustained the injury in training with Ulster last week and has been using crutches since.
Ulster Rugby confirmed the news, and revealed the 35-year-old has a hamstring tear.
"Following a scan, it has been confirmed that Rory Best has a hamstring tear and will be out of action for approximately 4-6 weeks."
It is likely he will miss Ulster's first two Champions Cup pool games in October against Wasps and La Rochelle.
The hooker may also miss three Pro14 games.
Ulster players Marcell Coetzee, Charles Piutau, Jean Deysel and Craig Gilroy are also out due to injury.