The champion jockey was riding out for Henry Cecil on Newmarket Heath when his mount whipped round abruptly, causing Fallon, who stayed on board, to twist his knee.
After partnering Poliziano into second place for Cecil in the opening Kennett Maiden Stakes at Newbury , Fallon decided to miss his remaining mounts.
Leaving the course for an X-ray examination at Addenbrookes Hospital in Cambridge, Fallon said: "I don't know what it is. I don't know if it is a cartilage or what, but it is worrying with Ascot round the corner.
"You can't ride with that sort of pain and I am going to have it looked at."
Cecil, the leading trainer by some distance at Royal Ascot, said: "I hope he is going to be alright for Ascot - if not, I am going to be scratching my head."
Also missing from Cecil's Ascot team after a gallops injury will be Daggers Drawn. The colt will be an absentee from next Thursday's Cork And Orrery Stakes, one of just three Royal Ascot races that Cecil has yet to win, after suffering a suspected fractured hind leg.
The chestnut, winner of Doncaster's Champagne Stakes at two and one-time favourite for the 2,000 Guineas, finished sixth of seven on his reappearance in the Craven Stakes and last of 18 in the Guineas.
Feeling Daggers Drawn had failed to stay, connections had planned to drop him back in trip, but plans will now have to be put on hold . "He was having a canter and he pulled up funny," Cecil said. "Hopefully it is not a permanent thing."
In addition to the Cork And Orrery, Cecil has to win the King's Stand Stakes and the Wokingham Stakes to complete a Royal flush of Ascot victories.Reuse content