Katie Walsh will be replaced by Nick Scholfield on Wonderful Charm in the Randox Health Grand National on Saturday after she fractured her upper arm in a fall in the Foxhunters' Chase over on day one of the meeting.
Walsh - who finished third in the 2012 National on Seabass - was unseated from Distime at The Chair, the third fence in the race run over the National course, and appeared to be kicked by another horse as she lay on the ground.
She was stretchered from the course into an ambulance and taken to hospital for further assessment.
A statement from Aintree read: "Katie Walsh, unseated from Distime at the third fence in the Randox Health Foxhunters' Chase today at Aintree, has a broken arm and will miss the ride on Wonderful Charm in Saturday's Randox Health Grand National.
"Walsh was assessed on course by Aintree's Racecourse's medical team. Subsequently, she has been transported to Aintree Hospital for further assessment for an arm injury and a scan."
Scholfield is attached to the yard of Wonderful Charm's trainer Paul Nicholls.
All horses involved in the Foxhunters' were reported to have returned safely after the race.