Reigning champion jockey Richard Johnson faces a spell on the sidelines after he broke his right arm in a fall at Exeter.
The 42-year-old was unseated from 11/8f Westend Story at the seventh fence of the Dare To Dream Chase on Tuesday.
The partnership had just regained the lead from eventual winner Breaking Waves when Johnson's mount got too close to the open ditch.
The leading rider was ejected out of the saddle and appeared to be kicked by the pursuing Fox Pro as he went to the ground.
When the TV camera returned to the scene of the fall around 20 seconds later, Johnson was still down as assistance arrived.
"Richard Johnson has yet to get to his feet after coming to earth from quite a height, when Westend Story got rid of him," commentator Martin Harris reported in-running.
Westend Story continued riderless alongside Breaking Waves (4/1), who survived a serious error at the second last fence before going on to win under Paddy Brennan.
Johnson, locked in battle with Brian Hughes to win the jockeys' title for the fifth time in succession, was taken to Exeter General Hospital to be assessed.
The injury was confirmed later in the day.
Johnson, replaced by Tom O'Brien on his two other mounts on the card, could miss the Cheltenham Festival, which begins on March 10.
