Sir Henry Cecil will not be at Glorious Goodwood on Wednesday to see Frankel go for his 12th successive win in the Qipco Sussex Stakes.
The world's highest-rated horse will face just three rivals - one of those being stablemate and pacemaker Bullet Train - in the Group One contest over a mile that he won last year.
Cecil, who has been having on-going treatment for cancer, said on his website, www.sirhenrycecil.com: "Unfortunately, I won't be at Goodwood on Wednesday as I have been undergoing some treatment.
"Although I am able to train my horses, I'm not quite fit to go racing yet, but I hope to be fully recovered in the near future.
"Everything has gone as we could have wished with Frankel since the Queen Anne Stakes. We gave him an easy time immediately after Royal Ascot before bringing him back into fast work.
"He has pleased us with his work and is going to the Sussex Stakes in good shape. We are taking one race at a time with him and are hoping that he can extend his unbeaten record."
Frankel's main test looks likely to come from Godolphin's Farhh, who has run such game races in defeat in the Prince of Wales's Stakes and the Coral-Eclipse.
Richard Fahey's Gabrial, who stepped up from handicap company to finish a close fifth in the St James's Palace Stakes at Royal Ascot, is the only other runner.