Having his first run since finishing second at Royal Ascot, Fellowes felt his four-year-old would have given the winner Withhold something to think about had misfortune not struck.
"He's ruptured a tendon, so his racing days are over," said Fellowes.
"We still have him, which is the main thing, but it will be a long road back for him. At least there'll be a future for him outside racing."
He added: "I'm gutted, he was our flagship horse and he would have been a very nice stayer next year.
"He was travelling so well that he may have given the winner a race.
"It's so disappointing, but he gave us a great day at Royal Ascot we'll never forget."