Robbie Henshaw will be out until the new year for Leinster after undergoing wrist surgery.
The Ireland centre had a procedure last week and is out of Saturday's URC clash with Ulster.
He will also miss his province's opening Champions Cup pool games against Racing 92 and Gloucester, and the St Stephen's Day meeting with Munster in the URC.
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Charlie Ngatai would be favourite to step into Henshaw's position against Ulster but suffered a shoulder injury in last weekend's 40-5 win over Glasgow and a decision on his fitness will be made later in the week.
The same applies for skipper Johnny Sexton, who injured his calf before Ireland's final Autumn series victory against Australia.
Better news for Leinster boss Leo Cullen is that James Lowe is back available for the first time this season after recovering from a calf injury.
Jordan Larmour will train fully after recovering from a foot injury while Harry Byrne, Rónan Kelleher and Ryan Baird all came through last weekend's fixture without incident on their return from injury.
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