But he admitted that Pat Eddery is "not a bad deputy" for his stable star, who faces 10 rivals in the Independent Purchasing Associates Stakes.
American rider Sellers bruised his left arm in a fall at Churchill Downs last Friday. He appeared to prove his fitness by partnering a winner in the USA on Saturday but suffered a relapse and has had to cancel his first trip to Europe.
Mitchell said: "It's very, very unfortunate. He had a fall on Friday but I spoke to him and he felt OK, just a little bit stiff. He had six rides on Saturday and rode one or two winners. But I spoke to him yesterday and he could hardly walk. He went to the track where he had physio and a whirlpool and rode in the first race but he literally had to be helped back into the weighing-room. I spoke to him and he said he would not be able to do justice to the horse, particularly after eight hours on the plane.
Running Stag should start a hot favourite as he is unpenalised for two Grade two wins in the United States.
n Hors La Loi III, the 17-length winner of the Supreme Novices' Hurdle at the Cheltenham Festival in March and a candidate for the Champion Hurdle there next year, was found to have broken a blood vessel when 15-length seventh to Homme Du Jour in the Prix Renaud du Vivier at Auteuil on Sunday. The Francois Doumen-trained gelding shocked connections with his poor performance. Hors La Loi III, 12-1 from 8-1 for the Champion Hurdle with William Hill, will have a short break before Doumen draws up a new plan for him.