Jack Conan had surgery in England yesterday to deal with a slightly complicated foot fracture.
The rehabilitation could take as long as four months from injury to getting back on the field.
The number eight will have to fight his way back up the pecking order at Leinster as Caelan Doris is given the room to grow towards the Champions Cup.
The 2020 Six Nations looks like it could just be out of reach unless Conan’s powers of recovery are better than expected.
Prop Ed Byrne’s return from a neck injury suffered in training ahead of the first round of the PRO14 League has been delayed.
"It has just been a bit slower than we thought," said coach Leo Cullen at the weekly press briefing this morning.
"He got a bang on his neck and it just hasn't quite settled down.
"We were hoping he was going to be involved over the last couple of weeks but it has been pushed out again.
"The break next week and hopefully he'll be good to go after that. But, again, we can't say for certain."