Alec Stewart, trainer of the great Mtoto, died at his Newmarket home on Wednesday after a long-running battle with cancer. He was 49.
Stewart trained Mtoto, ridden by Roberts, to Ascot glory in 1988
Stewart took out a training licence in 1983, following four years as assistant to Tom Jones.
He went on to establish a highly-successful partnership with South African-born jockey Michael Roberts.
"It is very sad news indeed as he was a great man and what you would call a true friend," said Roberts.
Stewart and Roberts enjoyed a fruitful association and landed several top races with some high-class performers.
The star turn among them was Mtoto, with whom they bagged the Coral-Eclipse Stakes in 1987 and 1988, the year in which the brilliant son of Busted also took the King George VI and Queen Elizabeth Diamond Stakes at Ascot.
"I owe him a lot as he helped me a great deal when I first came to the UK and he was always there for me during the 17 years I was here," Roberts added.
"He was a great trainer - in my eyes he was fantastic.
"You could get beat on one of his horses but the next morning he would be
just looking forward to that day's meeting, which was why I enjoyed riding for
him and why we got on so well.
"It is a huge shock for racing. My condolences go out to his wife Katherine and his children."
Fellow jockeys also paid warm tributes to Stewart.
Richard Hills, who won the Canadian International on Mutamam in 2001 for Stewart, said: "It's very sad. I have known him all my racing career.
"He was a great friend, a greater trainer and will be sadly missed. It's
Willie Carson, who also rode a number of winners for Stewart, added: "He was so young. It was a brave battle he had, and it's just very sad.
"He was a true gentleman. I had the highest respect for him."