The 5lb claimer, son of Grand National-winning trainer Martin Brassil, was partnering Demi Plie in the two-mile handicap hurdle when his mount came to grief at the third flight.
Brassil's agent, Ciaran O'Toole, said: "He's broken his leg just below the knee. He's being operated on today, I've spoken to his father.
"If it's a clean break (and) the surgery goes well with no complications, you usually look at six to eight weeks and then have a couple of weeks riding out. A lot can depend on the individual, some heal quicker than others."