Thorne was not included in the 23-man West Brom squad that jetted out to California on Thursday for an 11-day warm-weather training camp ahead of the start of the new Premier League season.
And Albion have confirmed that the 21-year-old, who spent time at Derby last season, remains keen to head back to Pride Park on a permanent basis, having handed in two transfer requests and also been the subject of a rejected bid in June.
"The club are in talks with Derby about George and while those negotiations continue it makes sense for him to remain at home," Albion head coach Alan Irvine told the club's official website.
"However, if no agreement is reached, George will be flown out to join the rest of the group in the US."
Thorne made 12 appearances for Derby last term and started the Championship play-off final defeat at the hands of QPR.
