Leinster will be without James Ryan for the restart of the Guinness Pro14 campaign and the province's Champions Cup quarter-final against Saracens.
The Ireland lock suffered a shoulder injury in training and underwent surgery that will keep him out for up to three months, Leinster said in a statement.
"Ryan suffered a shoulder injury in training before the recent break and had a procedure to address the issue," the Pro14 champions said.
"He will be out of action for up to 10-12 weeks."
Leo Cullen’s side are due to play Munster in the Aviva on 22 August, while they are scheduled to host Saracens on 19 September in a Heineken Champions Cup quarter-final.
Ryan, 24, will then have to prove his fitness to Ireland head coach Andy Farrell, whose side are set to face Italy on 24 October and France a week later to conclude the 2020 Six Nations.
Winger Adam Byrne has also been ruled for out 12 weeks with a hamstring injury, which also required surgery.
Prop Peter Dooley should return to training in two weeks following a shoulder injury, while fellow front-rower Vakh Abdaladze is out with a back injury.
Ireland flanker Dan Leavy "is continuing to integrate gradually into rugby training after a significant multi-ligament knee injury".
Wing James Lowe is back in full training after following all the coronavirus-prevention guidelines upon his return from New Zealand.