The Italian had partnered the star miler to victory in last week’s Qatar Sussex Stakes at Goodwood.
But the three-year-old colt picked up an injury in the race.
He successfully underwent surgery at Newmarket Equine Hospital for a hairline fracture to a right-hind cannon bone.
Dettori, on holiday in Sardinia, exclusively told the Express: “It’s absolutely devastating news.
“I had so much to look forward to – the Breeders’ Cup. It was amazing last week at Goodwood.
“We had just got him in the right place, it’s a terrible loss, not just for me, a terrible loss for racing – a champion like him.
“Potentially, he could have been the best of the year. A truly great horse and the best two-year-old I’ve ever ridden.
“He was the highest-rated juvenile of last year – the same as Frankel when a two-year-old.”
Dettori, who rode Too Darn Hot in all his nine races, added: “I’m so sad for everybody involved – Maisie (Hainey) who rides him at home, the Lloyd Webbers (owners) who are great supporters of racing and Sheikh Mohammed.”
Simon Marsh, racing and bloodstock manager for owners Lord and Lady Lloyd-Webber, said: “Too Darn Hot has undergone emergency surgery to his right-hind cannon after X-rays revealed a hairline fracture. I am glad to say that he has come safely through surgery.
“The injury is not life threatening and he should make a full recovery. His future now lies at Dalham Hall Stud in Newmarket where he will recuperate and stand at stud for the 2020 season.”
A son of Dubawi, Too Darn Hot was unbeaten in four starts as a juvenile, culminating in the Darley Dewhurst Stakes at Newmarket.
An injury-delayed return this season came in the Dante Stakes at York, where he was second to Telecaster, before finishing second in the Irish 2,000 Guineas and third in the St James's Palace Stakes.
He recaptured the winning thread at the top level with a smooth triumph in the seven-furlong Prix Jean Prat at Deauville, and followed up in equally impressive fashion back at a mile at Goodwood.
Marsh added: “The Lloyd Webbers and I are very sad we won't see this champion on the racecourse again.
“He has given us all so much excitement winning three Group One races over the last 12 months, culminating in his brilliant win last week in the Group One Qatar Sussex Stakes.”