Doyle was diagnosed with the problem during a team-break to America last month. The 21-year-old required an emergency operation to have his appendix removed and as he was unable to fly back to the UK immediately he remained in the States with his parents in Texas.
"Conor flew back on Wednesday and it'll be lovely to see him again," Rams boss Nigel Clough told the club's official website. "He's been doing some rehabilitation work back in the States but his return will allow us to assess him properly for the first time since the operation, and we'll take it from there.
"We've been in touch with him over the internet in the last few weeks so the medical team have been able to communicate with him as normal, and they seem to be happy with his progress.
"We'll still be guided by the doctors with regards to when he can rejoin training and eventually return to action for the Under-21s, but hopefully it won't be too long."
Doyle, who had scored four goals in five games for the Under-21 side this season, has made one start and a further appearance from the bench this season for the first-team.