Jamie Codd suffered a partially collapsed lung in a fall at Limerick on Friday and is expected to be out of action for a month.
The leading amateur rider was riding Triumph Hurdle winner Farclas for Gordon Elliott when taking a particularly bad fall at the third flight in the Irish Independent Hurdle.
Dr Jennifer Pugh, chief medical officer for the Irish Turf Club, said: "Jamie suffered a partially collapsed lung and is due for a scan later today (Saturday), but we'd expect him to be discharged afterwards. He's looking at about one month on the sidelines.
"He was sitting up having a cup of tea last night and joking about how his nose didn't escape without injury either, so he certainly hasn't lost his sense of humour anyway."
Keith Donoghue's fortunes took a turn for the worse on the same afternoon. He had ridden a Grade One winner on Hardline but is now set to be out for a month too with a facial injury, after falling from Burren Life in the Tim Duggan Memorial.
Pugh said: "Keith suffered a facial and nasal fracture, but he'll be fine. He's looking at a month or more."
She added: "Bryan Cooper had a concussion here at Leopardstown on St Stephen's Day, and there's obviously a mandatory six days standing down period for that. Sean O'Keeffe suffered a concussion as well."