Winner of last season's Cesarewitch, the five-year-old mare began this campaign on a winning note in the Sagaro Stakes at Ascot but has since found things tough.
Morrison had intended aiming her at Friday's Doncaster Cup, in which she was fifth last year, but has instead opted for the fillies' St Leger.
"It was a difficult decision," said the East Ilsley trainer.
"We always felt the Doncaster Cup was probably the right race for her, but, with the slower ground, the fillies' race didn't look as strong.
"She's had a bit of a break. We sent her away for a couple of weeks after she ran flat at Goodwood.Sweet Selection last won at Ascot in May (PA)
"I don't think she's really sparkled since she won the Sagaro. The Gold Cup was a very challenging race for her.
"We just hope for a nice run and we'd like to see her bounce back. She owes us nothing. I'm more concerned about the ground."
The race features five horses who ran in the Group Three Lillie Langtry Stakes at Goodwood, won by Godolphin's Endless Time.
They are runner-up Dubka, Melodic Motion (third), Natural Scenery (fourth), Wild Irish Rose (fifth) and Harlequeen (ninth).
Ralph Beckett was pleased with Melodic Motion's effort considering his stable was under a cloud shortly afterwards.
"I thought Melodic Motion ran really well at Goodwood last time," said the Kimpton-based trainer.
"I'm not blaming the ground or anything for her winning run ending that day.
"The only thing I would say is that the following week we then shut down and didn't have any runners for two weeks and she was one of the last ones before that to run well."
Beckett also runs Alyssa, who won a Listed contest at Ascot last September after having been well beaten in this race, won by her stablemate Simple Verse, 12 months ago.
The daughter of Sir Percy was third to Nearly Caught in the Coral Marathon at Sandown in July on what was only her second start of this campaign.
Saeed bin Suroor sends Natural Scenery back into Group company following her fine effort to finish third behind Nakeeta in the Ebor at York.
"Natural Scenery ran very well at York on her latest start and is in good form," the Newmarket trainer told http://www.godolphin.com.
"She is ready to go again and this looks to be a nice race before we look at taking her out to Australia."
Wild Irish Rose is trained by Aidan O'Brien, whose son Joseph is responsible for the other Irish raider, Detailed, winner of three of her last four races.