Doyle was rushed into hospital during the club’s mini break to Miami last week.
He has now returned to his family home in Dallas, Texas, but he is not allowed to fly back to England for a few weeks.
“We are in the hands of the doctors,” said manager Nigel Clough. “It was a particularly nasty appendicitis. He is not allowed to fly externally for maybe three or four weeks because of the nature of the operation.”
Clough said Doyle suddenly felt unwell and started being sick.
“We thought it was food poisoning at first because he was being sick,” Clough added. “Then he was rushed into hospital at about 2.30 in the morning in an awful lot of pain.
“The only good thing is he’s at home! He was allowed to fly internally and went home to Dallas, where he will spend some time recuperating.
“Had it happened to one of the English lads, we would have had more of a problem.”
An appendix operation was not the birthday present Doyle wanted.
He was 21 on Saturday and members of his family had planned to spend a few days with him at Derby’s hotel in Miami.
Doyle has had a handful of first-team appearances this year and also leads Derby County’s U-21 squad in scoring.