Robbie Henshaw has been ruled out of action until January after undergoing surgery on a wrist injury.
Leinster head coach Leo Cullen confirmed the centre "underwent a procedure" last week which will see him miss the opening rounds of the Heineken Champions Cup as well as three interprovincial derbies.
Henshaw was restricted to just five minutes of action for Ireland during the autumn internationals due to a hamstring injury.
"It's disappointing to lose him, particularly losing Ciarán Frawley [to injury] as well," said Leinster senior coach Stuart Lancaster.
"It's not too serious an injury but it needed doing. It’s not a major thing for him. I'm pretty confident he’s going to be back early in the new year.
"Charlie Ngatai is more than able to play at 12 and we have got other options as well. It’s disappointing for Robbie but he’ll be back soon enough."
Leinster will make a final call on Johnny Sexton and Ngatai's fitness later in the week as they prepare to face Ulster at the RDS on Saturday.
Sexton hurt his calf during the warm-up ahead of Ireland's game against Australia while Ngatai suffered a shoulder injury in last Saturday's win over Glasgow Warriors.
There was positive news on James Lowe and Jordan Larmour, who are back in full training after recovering from calf and foot injuries respectively.
Lowe has yet to feature this season while Larmour has been sidelined since early October.
"James and Jordan have had a tough run with injuries," said Lancaster. "We're missed them, especially on the training field. Their energy is infectious. They're both brilliant lads who bring a huge amount of energy to the group.
"They’ve been out for such a while now that it feels like two new players arriving into the team. James was at his vocal best today, looking sharp. It’s exciting to have them back."
Leinster sit top of the BKT United Rugby Championship after eight successive victories, eight points clear of second-placed Ulster, who were 20-10 winners on their last visit to the RDS in November 2021.
Rónan Kelleher and Harry Byrne came through the Glasgow game unscathed having returned from hamstring injuries, as did Ryan Baird after completing his return-to-play protocols.
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