A final decision on Magician's participation in the St James's Palace Stakes will be made on Sunday after the colt sustained minor bruising following a knock on Thursday, Coolmore said.
Impressive winner of the Irish 2,000 Guineas, the Aidan O'Brien-trained colt had previously won the Dee Stakes at Chester's May meeting.
Mars, sixth in the Investec Derby at Epsom, is on standby to substitute for Magician should he not run.
A statement on www.coolmore.com read: "Magician sustained minor bruising after receiving a knock yesterday.
"Aidan is very happy with Magician today and he is 100% sound.
"A final decision will be made on Sunday whether or not he will run in Tuesday's St James's Palace Stakes.
"Should he not run, Mars will take his place and in the event of that happening Indian Chief will run in the Tercentenary."
Confirming running plans for the O'Brien two-year-olds next week, the statement went on: "Two-year-old plans for Royal Ascot are as follows: Stubbs, Sir John Hawkins and War Command are all probables for the Coventry and Fountain Of Youth goes for the Windsor Castle.
"Bye Bye Birdie runs in Mallow (Cork) on Sunday after which a decision will be made on the Queen Mary.
"Coach House is a probable for the Norfolk Stakes and Wonderfully is likely to represent us in the Albany.
"We plan to run both Friendship and Adeste Fideles in the Chesham Stakes."