Ulster captain Rory Best is a doubtful starter for his side's upcoming Pro 14 clash against Ospreys at Kingspan Stadium following a hamstring injury.
The 35-year-old picked the strain during the training session on Tuesday and has undergone scans to know the extent of the damage.
Meanwhile, Iain Henderson is set to miss the game on Sunday after failing to recover from a knee injury that he sustained in the game against Munster. However, the 26-year-old is hopeful of making it to Ireland's summer tour of Australia.
The winner of the Belfast encounter on Sunday will seal the seventh and final qualifying spot for the next season of European Champions Cup.
Ulster managed a fourth finish in Conference B of the Pro 14 table with 12 wins from 21 games.