Ulster winger Jacob Stockdale has been ruled out for six to eight weeks with a grade 2 hamstring strain sustained in training last week.
Stockdale, 22, was unlikely to feature in the first couple of Pro14 games after playing a significant part in Ireland's summer tour to Australia.
The Ireland back will now be absent until at least late September.
Fellow backs Louis Ludik, Luke Marshall and David Busby will also miss the early part of the season with injuries.
South African Ludik faces several months on the sidelines after picking up an injury on the final day of last season, while Busby and Marshall are recovering from ACL tears.
Centre Marshall tore his ACL in his right knee in the Pro14 play-off against Ospreys for a place in the Champions Cup in May and is not expected to feature until the new year.
The setback for Stockdale - who scored 20 tries for Ulster and Ireland last season - adds to Ulster's growing back-line injury list, although Craig Gilroy and Rob Lyttle can fill in on the wing.
Wingers Andrew Trimble and Tommy Bowe retired at the end of last season while Charles Piutau departed to play in the English Premiership with Bristol.
Short-term signing Henry Speight, who can play full-back or on the wing, will train with his new team-mates for the first time this week after the Australia back arrived from the Brumbies.
Speight will return to Australia on 31 December in advance of the 2019 Super Rugby season.
Ulster will begin their Pro14 campaign at home to last season's beaten finalists Scarlets on 1 September.