The retained rider to owner JP McManus took a tumble when his mount Cook Islands was brought down by Wonderoftheworld at the final flight in the Irish Stallion Farms EBF Novice Hurdle.
An X-ray showed no breakages and it was originally hoped the top Irish jump jockey just had soft tissue injuries that would keep him out of the saddle for about a week.
However, further medical investigation revealed the true damage.
"It showed up on an MRI on Friday. I had an X-ray on Tuesday. It's probably another three weeks I'm looking at," said Geraghty.Barry Geraghty has ridden five winners from 20 rides this injury-plagued summer. (PA)
"I'll be seeing my surgeon in another fortnight and we'll have a better idea then, but it's looking like that.
"It's obviously not great being on the sidelines again, but hopefully I'm putting my bad luck behind me and I'm hoping for a clear run."
It is another blow for Geraghty, who only returned from a spell on the sidelines at the Galway Festival after being off since April with a broken arm sustained in a fall from Minella Foru in the Irish Grand National at Fairyhouse.
He was forced out of this year's Cheltenham Festival when suffering lung and rib injuries in a fall at Kempton in February. He also broke his other arm in a fall at Market Rasen last July.