Tom Marquand was first choice for the up and coming Ed Walker-trained sprinter but Murphy will have the mount as Marquand is required to ride Nahaarr by William Haggas in the G1 race.
Marquand had ridden Starman in all four of his starts last year, in which he recorded three victories, but Murphy took over in the G2 Duke of York Stakes at York where Starman defeated Nahaarr by a neck.
"Tom would have ridden Starman had he been available but I've just heard today that he will almost certainly be riding Nahaarr so Oisin will keep the ride going forward," said Walker.
Starman, unraced as a two-year-old, progressed rapidly at three, winning a Listed race on his third start to earn a run in the Qipco British Champions Sprint on soft ground at Ascot in which he suffered his first defeat, finishing 14th.
He was promoted to the head of the betting for the Diamond Jubilee Stakes, part of the Qipco British Champions Series, after an authoritative display at York.
"He is a massively exciting horse," said Walker. "He's done nothing wrong. He is a big imposing horse with that presence, a real head turner in the string. He's obviously very good as well. Everything has gone well since York so I am very pleased at this stage.
He added: "Nahaarr got pretty close to us at York and Dream Of Dreams is a G1 winner. I kind of feel that they both are probably at their best – I hope I don't regret saying this – with a bit of dig in the ground so I think if it's good, fast summer ground it will play into our favour slightly."
Walker has one Royal Ascot victory to his name but has other strong chances this year, including the filly Primo Bacio, who is in the same David Ward ownership as Starman and was a three-length winner of a Listed race also at York's May meeting. She is targeting the G1 Coronation Stakes, part of the Qipco British Champions Series.
"She has really blossomed into a good filly," said Walker. "She kind of had her excuses last year. She has improved a ton from two to three and her work all spring has been high quality.
"She's not put a foot wrong, was very unlucky at Newbury but in a funny kind of way I am glad she didn't get a clear run and go and win the Fred Darling because we probably would have been forced to back her up quite quickly in the 1,000 Guineas. We had a conversation about going and quickly canned it.
"That break freshened her up and she was spot on for York and there is a similar gap to Ascot so hopefully we are producing her in the same form as she went to York. It's going to be a very different test. There are lots of questions to answer there."