Buick had the choice of Romsdal and Forever Now, who are both trained by his boss, John Gosden.
Romsdal finished third in the Derby and was last seen when well beaten in the King George at Ascot, while Forever Now won the March Stakes at Goodwood on his latest outing.
Despite that victory, Buick has decided to partner Romsdal.
Buick said: “In the end I just gave the nod to Romsdal on having the better overall level of form and that when I did ride him we were only beaten a nose by Orchestra in the Chester Vase over 12 (furlongs), staying on strongly.
“I’ve made my decision — I think it’s the right one so here’s hoping.”
Frankie Dettori has been confirmed as the rider of Forever Now. It has been a difficult week for the popular Italian after he lost the ride aboard last year’s Prix de l’Arc de Triomphe heroine Treve.
Al Naamah, the Andre Fabre-trained filly who cost an five million gns, then finished only sixth in the Prix d’Aumale at Chantilly on Tuesday.
Dettori’s agent Ray Cochrane said: “Frankie will ride the John Gosden horse (Forever Now) in the St Leger, which is great.
“It’s not all doom and gloom.
“Last month Frankie rode a Group One winner in France for James Fanshawe (Ribbons) and I’m sure Hayley Turner (previous jockey) wasn’t too pleased, but that’s the way it goes. It’s swings and roundabouts.”