Santiago will lead a three-strong team for Aidan O'Brien in the Pertemps St Leger - with Tiger Moth set to head elsewhere.
Irish Derby winner Santiago is set to be joined at Doncaster on Saturday by Dawn Patrol and Mythical.
Tiger Moth, a fast-finishing second to Santiago in the Curragh Classic, has options at home this weekend - or could even join stablemate and Investec Derby winner Serpentine in the Grand Prix de Paris.
"We're going to run three at Doncaster - Dawn Patrol, Mythical and Santiago - that's the plan at the moment," said O'Brien.
"Tiger Moth could either go to Leopardstown or Longchamp, the Grand Prix with Serpentine or the Kilternan (Paddy Power 'Is It 2021 Yet?' Stakes) at Leopardstown."
O'Brien confirmed recently that Frankie Dettori is likely to be part of his team for the Classic once again - the two having teamed up for St Leger success in 2005 with Scorpion.
However, final riding plans are still to be confirmed - with William Buick, who had been mooted, likely to be in Ireland to ride Ghaiyyath at Leopardstown.
"We have Frankie booked, obviously, and I think Adam Kirby might be booked," said O'Brien.
"But what happens is the horses will all have their tests in the next few days, and we'll confirm everything after that."