Andrew Thornton faces six weeks on the sidelines after his fall at Southwell on Sunday.
Thornton was dislodged from Robbie in the Mansfield Chad Handicap Hurdle at the final flight when still holding every chance.
He left the course in a wheelchair and only got to learn the full extent of his injuries on Tuesday.
"I've seen Dr Fog and had a scan and an X-ray, and he's going to perform some keyhole surgery on it later today," said Thornton.
"He reckons I'll be out for six weeks. He just said it needs cleaning up, he was pretty vague about what the actual problem was.
"I'm in no pain. He drained the fluid off my knee and I'll know more tomorrow.
"Hopefully it sounds like a straightforward procedure."
