David Marriott told CNN that Knox never agreed to return to Italy and that there was "no requirement she be there."
She is not required to attend the trial and can be represented by her lawyers, though it's possible Italy can request an extradition.
A lawyer for Knox, Luciano Ghirga, was also quoted in the Italian newspaper La Nazione on Saturday saying that Knox and her defense team recently met in the US and confirmed that the American won't step back on Italian soil for the new trial.
Knox told CNN in May that she wasn't sure if she'd return to Italy.
"It's a really complicated question," Knox said. "I mean, I'm afraid to go back there. I don't want to go back into prison."
The Italian Supreme Court ordered the new appeals trial in March for Knox and her then-boyfriend Raffaele Sollecito in the murder of Kercher.
Kercher, a British student studying in Perugia, was found stabbed to death in the home she shared with Knox in November 2007.
Knox and Sollecito were both originally found guilty of the murder in 2009 but were acquitted two years later for "lack of evidence".
Knox spent four years in an Italian prison before returning home to her family in Seattle.
The high court said that judges overlooked evidence that may point to a prosecution theory that Kercher was killed during a group sex game gone wrong.
One person is currently in prison for the murder.
Ivory Coast-born Rudy Guede received a 16-year sentence in a separate trial.
The new appeals trial begins in Florence on Sept. 30.