Dettori replaced regular jockey Joey Sheridan in last month's Diamond Jubilee Stakes because 7lb claimers were barred from riding in Britain until June 22 under the sport's coronavirus restrictions.
Sceptical, who was bought unraced for just £2,800 in August last year by owner James McAuley and put into training with Denis Hogan, has won four of his six starts to become Ireland's top-rated sprinter.
He finished third at Ascot, hitting the front a furlong out only for Hello Youmzain to battle back to beat Dream Of Dreams and Sceptical in a photo-finish.
Sheridan partnered Sceptical on the gallops this morning but Dettori will be back in the saddle at Newmarket in one of the few G1 races he has yet to win.
"Frankie will ride him as he said coming off the course at Ascot that had he known him he definitely would have won the Jubilee," said McAuley. "He said the July Cup is one of the few Group 1 races he has never won and he felt he could win it on Sceptical."
Sceptical lined up in the Diamond Jubilee 12 days after winning the Woodlands Stakes at Naas on his first start on turf. His team had been considering also targeting the King's Stand Stakes four days earlier.
"He was perfect this morning and has had a much better preparation than he did for Ascot," said McAuley. "It took him four or five days to get over Naas, that's why we didn't go for the King's Stand. Everything was a bit rushed. He came out of Ascot far better and I am far happier sending him over this time than before.
"Frankie admitted he got it wrong at Ascot and went too soon, but I wouldn't blame him because when he hit the front I was delighted and thought he would keep going. If the ground stays quick we are hoping for the best."