Buzz - winner of the Cesarewitch and a leading fancy for the Stayers' Hurdle at the Cheltenham Festival - suffered a suspected fractured pelvis on Nicky Henderson's Seven Barrows gallops during morning exercise ahead of his Ascot run.
The field is now reduced to eight, which includes last year's first two - Paisley Park and Thyme Hill - while Thomas Darby and Ronald Pump seek their first top-level success.
While Henderson is naturally distraught that last month's Coral Ascot Hurdle winner will now be sidelined, he is still doubly represented with On The Blind Side and Champ.
Champ reverts to the smaller obstacles on his first competitive appearance since being pulled up in the Cheltenham Gold Cup nine months ago.
Henderson admitted: "He has had his issues, particularly his jumping, really. But his jumping was suffering because he had a bad back. He has had back surgery and we are very happy where he is with his work.
"We just felt that with his jumping in particular, bearing in mind that if he goes over fences, he has to go left-handed - not left-handed over hurdles - this was a good spot, especially since I can't find a left-handed chase that suits him.
"So, our main objective, while he has got to try to win the Long Walk Hurdle, if he does run well, we might just say, 'hang on a minute, are we not better off looking at the Stayers' Hurdle rather than the Gold Cup?'
"The intention will be to go back chasing, but you never know.
"Above all, I am trying to boost his confidence and get him to enjoy himself - which he will do, I hope, over hurdles. It is going to be a very tough race."
Henderson's second string is On the Blind Side, who was runner-up to Thomas Darby on his seasonal debut in the Long Distance Hurdle at Newbury last month.