Barry Geraghty was taken to Liverpool Hospital with a suspected fracture to his leg after a fall in the Topham Chase over the Grand National fences.
Geraghty was riding Peregrine Run and was still in contention when coming down late on.
After initial assessment, he was taken to hospital in an ambulance, missing the winning ride on Champ in the Grade One Doom Bar Sefton Novices' Hurdle, which went to Mark Walsh.
Champ's trainer Nicky Henderson said: "We have a nasty feeling that Barry has broken his leg, which would be tragic."
Grand National 2019 Pinsticker guide: Every runner, star ratings, rider colours and big race tips
Geraghty is due to ride Gold Cup runner-up Anibale Fly in Saturday's Grand National. Walsh, like Geraghty retained by owner JP McManus, is currently due to ride Regal Encore.
Mark Enright was also taken to hospital following his tumble from Call It Magic at the Canal Turn.
Enright is also scheduled to ride in the National, on Gordon Elliott's Noble Endeavor.
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