Jockey Robbie McNamara has undergone surgery after suffering serious injuries in a fall at Wexford on Friday.
McNamara was transferred to the Mater hospital from Wexford General Hospital and Culloty told the Racing Post that the rider had undergone surgery to remove a haematoma pressing on his spinal cord.
"My heart is thinking of Robbie McNamara at the moment," Culloty told Channel 4 Racing before the Grand National. "He is in surgery at the minute. As well as eight broken ribs and a punctured lung, he fractured his T11 vertebrae and there is a bit of spinal damage.
"There is a big haematoma on his spine pressing on his spinal cord and they are trying to remove that at the moment. My prayers are with him."
The jockey was due to partner last year's Cheltenham Gold Cup winner Lord Windermere in Saturday's Crabbie's Grand National at Aintree
McNamara, who rode a double at last year's Cheltenham Festival as an amateur before turning professional earlier this season, was partnering 14-1 shot Bursledon when he came to grief four flights from the finish in the first division of the Cahore Point Handicap Hurdle.
He was taken to Wexford General Hosptial, where he underwent a CT scan, before being transferred.
Dr Adrian McGoldrick, chief medical advisor for the Irish Turf Club, said: "Robbie has chest, abdominal and spinal injuries and has been transferred to Mater (Hospital).
"I have spoken to his family and we have agreed there will be no further comment until Sunday, by which time he will have undergone further scans."