"She seems all right, I can't fault her, really," Macdonald, who trains with his son-in-law Andrew Gluyas, told the Herald Sun.
"She missed the start and travelled four-deep the trip - she had a horrible run.
"She might have jarred up just slightly, but I'm not blaming that. We'll put it down to circumstances."
It was an uncharacteristic effort from the Southern Image 5-year-old, who has finished outside the top five in only four of her 24 starts.
While it's not unusual for mares to throw up a bad performance, it's not in Southern Speed's nature.
"That's the thing that concerns you a little bit, she just doesn't do it," Macdonald said. "We certainly expected her to go much better."
Macdonald struggled to understand the effort straight after the race, but his mood had lifted yesterday.
As TAB Sportsbet wound her price for the Caulfield Cup out to $17, he remained optimistic.
"Of course she can bounce back," Macdonald said.
"They can all run a bad race, her run might not have been much worse than Manighar's was a fortnight ago.
"Let's see what happens, it's onwards and upwards from here."
Southern Speed ate up well after the race and will have a light week on the training track as she prepares for the 2400m classic. Macdonald said Glen Boss still has first right of refusal on the Caulfield Cup ride.