A fall has ruled Harry Cobden out of the Randox Grand National after he suffered a fall earlier on the Aintree card.
The leading rider came off Lucky One in the second race of the day, the Grade 1 Betway Mersey Novices' Hurdle.
Cobden, 22, stable jockey to 11-time champion trainer Paul Nicholls, has been taken to hospital for assessment.
It was reported he was knocked out when he hit the ground, but regained consciousness and was able to walk to the ambulance.
An update from the British Horseracing Authority on Twitter read: "Following a fall in Race 2, Harry Cobden has been stood down by medical officers and will not take up his rides for the remainder of the day. Confirmation of rider changes will follow in due course."
Cobden was due to partner Give Me A Copper in the National, a horse part-owned by Sir Alex Ferguson.
ITV Racing reported that Sean Bowen will now take the ride on the 11-year-old, around 40-1 with the bookmakers.
Favourites took the first three races – Hometown Boy (6-1f), My Drogo (5-4f) and Shishkin (1-8f).
The latter, exceptionally short in the betting, failed to impress in the Grade 1 Doom Bar Maghull Novices' Chase.
Jockey Nico de Boinville said he was "idling" going to the final fence and suggested he would do better off a stronger gallop.
