Packwood suffered a broken fibula and tibia following an accidental collision with Leeds striker Luciano Becchio during the FA Cup third-round tie at Elland Road.
An update on the 19-year-old on the Blues' official website said: "Will was successfully operated on in Leeds General Infirmary on Sunday morning and it is hoped he will be transferred to Spire Little Aston Hospital in Sutton Coldfield tomorrow (Monday) afternoon to begin his recovery.
"The initial indication is that Packwood is likely to be sidelined for at least nine months."
Birmingham have been inundated with good will messages for the American.
He said via Twitter: "Thank you to everybody for all the support I've received! It means so much to me to have you all behind me."