Adayar, who beat Hurricane Lane in the Epsom Derby, will not renew his rivalry, having been one of 14 horses to be withdrawn from the final Classic of the season.
This has seen the Irish Derby and Grand Prix de Paris victor Hurricane Lane strengthen his credentials for the Doncaster race, with Planet Sport Bet making him the 4/5 favourite for the encounter.
Meanwhile, trainer Charlie Appleby is expected to take Adayar to the Prix de l'Arc de Triomphe on October 3. Adayar is currently 4/1 to win the Longchamp contest.
Two other horses of interest are Johnny Murtagh's vastly improving and prolific winner Ottoman Emperor - victor in the Group Three Gordon Stakes at Glorious Goodwood - and Lone Eagle, who was second in the Irish Derby for Martyn Meade.
Aidan O'Brien, a six-time winner of the St Leger, still has eight in contention this year. These include Sir Lamorak (22/1) and Sir Lucan (14/1). The latter was second to Ottoman Emperor at Goodwood and then fourth in the Great Voltigeur - in which stablemate The Mediterranean was runner-up, and may again re-oppose.
Murtagh says that Ottoman Emperor, currently 13/2 second favourite behind Hurricane Lane, is in fine form as he prepares for the St Leger.
"That is the plan so far - we've had no problems, touch wood," said the Kildare trainer.
"I've not been speaking much to the OTI people (owners) since after Goodwood - (but) the English St Leger is the next stop.
"That last run was good, and the form of that race stood up at York."
Murtagh admits requisite stamina remains the great unknown as Ottoman Emperor bids for the longest Classic, having been beaten just once in his career so far - on debut in a Dundalk maiden back in March.
He added: "I think he deserves his chance in the St Leger - whether he stays the mile and six is a question mark, but we'll have to pay to find out!
"You won't find out until the day, when he goes past that mile and a half.
"But he's a relaxed horse, and he gives the impression that he will (stay)."