Derby winner Masar has been ruled out of the Coral-Eclipse at Sandown with swelling in a leg.
The Charlie Appleby-trained Masar was found to be sore following exercise on Friday and all plans are now on hold.
"Unfortunately after exercise this morning, when he was cooling out, he had inflammation and soreness in his right off-fore," Appleby said.
"It was under observation during the course of the day and we felt this afternoon the soreness and the inflammation were still there.
"After speaking to His Highness Sheikh Mohammed, his first concern was to make sure that we were doing what was right for the horse and best for the horse.
"He is under veterinary observation and until the inflammation comes down, we won't be any wiser as to what extent of an injury we are dealing with. It will be over the course of the next 48 hours.
"First and foremost we need to get the inflammation down - that is what our priority is. It looked like it was a bang, but until the inflammation comes down, you don't know."
While Masar misses out, Godolphin and Appleby will still be represented on Saturday (Sunday AEST) by 2016 winner Hawkbill, who will be ridden by William Buick.
"He's a previous winner of the race and he's there genuinely on his merits," Appleby said.
"He's in good order and if he's on his A-game, I think he'll test the three-year-olds, which is what it's all about."
Masar's absence means the John Gosden-trained Roaring Lion looks likely to sent off favourite for the Group One feature.
He finished third in the Derby and fifth in the 2000 Guineas behind Eclipse rival Saxon Warrior, one of three runners for trainer Aidan O'Brien.