Michael Lowry has been ruled out for 6-8 weeks after undergoing ankle surgery.
Ulster Rugby's medical performance team issued an update on Tuesday ahead of Friday's Guinness PRO14 game against Cardiff Blues at Kingspan Stadium.
"Following his return from South Africa, Michael Lowry underwent ankle surgery and will be unavailable for 6-8 weeks," the statement read.
"The following players have sustained recent injuries and are also unavailable for selection this weekend: Rob Lyttle (back) Tommy O’Hagan (knee)."
Ulster fell to a heavy 63-26 defeat to the Cheetahs before bouncing back with a good 42-17 win over Southern Kings.
Lowry suffered his injury in the Cheetahs game in Bloemfontein and is set to miss some key games.
Ulster play Bath, Clermont and back-to-back games against Harlequins before Christmas as well as inter-pro derbies against Munster and Leinster.
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