Jockey Peter Toole is in a medically-induced coma after sustaining head injuries at Aintree on Saturday.
The 22-year-old rider suffered bleeding on the right side of his brain when 100-1 shot Classic Fly fell at the first fence during the Maghull Novices' Chase.
Toole was taken to Fazakerley Hospital, near Aintree, but was later transferred to the neighbouring Walton Centre for Neurology and Neurosurgery.
Trainer Charlie Mann, to whom Toole is attached, said: "He's in an induced coma and he won't be brought out until Monday at the earliest.
"He has bleeding on the right-hand side of his brain. He also dislocated a shoulder which has since been put back in place.
"He's on a ventilator, and it's not great - but we won't know any more until Monday."
Toole has ridden 30 winners this season and was due to have six rides at Wincanton on Sunday.
Weighing-room colleague David Crosse added: "I've spoken to Peter's dad who said he remains stable, but with no improvement.
"He's going to be sedated for 24 hours."