Derby striker Conor Doyle is recovering at home in Texas after undergoing an emergency appendix operation.
The United States Under-23 international was diagnosed with appendicitis during a team break to America and is unable to return to the UK for at least a month.
The Rams' medical team do not currently have a timescale for the 21-year-old's return to fitness, but remain hopeful he will be back before Christmas.
Boss Nigel Clough told the club's official website: "Initially when we were in America he was sick quite heavily and we thought it was food poisoning, but when it got to the point he was actually coughing up blood we knew it was something far more serious.
"We're not sure when we expect Conor to be back but we're probably looking at him returning in November and playing at some point in December.
"Our medical team haven't dealt with any cases like this here before so we're in the unknown to a degree so we'll be guided by the doctors and go from there."