Ante-post favourite Ask Me Early is a doubt for his major festive assignment after a fall.
The grey, 5-1 with the sponsors for the Coral Welsh Grand National, came down during a schooling session.
Jockey Sean Bowen went to hospital as a precaution after an accident at Harry Fry's stable on Wednesday morning.
"As Ask Me Early is favourite for the Coral Welsh National I wanted to let people know that he took a heavy fall with Sean Bowen while schooling over our fences this morning," the trainer wrote on his website.
"It was a big relief to see horse and rider get to their feet afterwards. Sean is a bit stiff and sore, so after being checked over at Yeovil hospital Sean decided to miss his rides at Newbury today. Hopefully he will be back in a few days.
"It was a shock to see Ask Me Early come down because normally he is the best of jumpers. He seems OK but it is too early to say if he escaped unscathed so we will be monitoring him in the days ahead."
The seven-year-old has had half of his eight career starts under rules at Chepstow.
He has won three of them, including his most recent outing earlier this month.
Under Bowen, he finished alone in a novices' hurdle – but was well clear when his main rival fell at the final flight.
Only on Tuesday had the jockey celebrated riding a winner at every racecourse in the UK.
He achieved the feat by partnering odds on shot Brorson to victory at Catterick.
The novice has helped contribute to trainer Martin Keighley's excellent season – and he needs just one more winner for his best ever tally.
Bowen confirmed via social media his plans to take a short break from competing while he recovers from the fall.
He was booked to ride four horses at Ffos Las on Thursday and Thomas Darby in Ascot's Grade 1 Howden Long Walk Hurdle.
Bowen's brother James is due to team up with the Nicky Henderson-trained On The Blind Side, a stablemate of ante-post favourite Buzz.
