Midfielder Owen Garvan has been diagnosed with post-viral fatigue syndrome and is expected to be out of action for several weeks. The Irishman suffered a bout of shingles in pre-season and has failed to fully recover.
Town's head of sports science Dave Williams told the club site: "Owen has been undergoing tests at the Olympic Medical Institute in Northwich Park, London this morning.
"He has seen the doctors there and they have taken some more blood tests and asked us to organise an ultrasound of his abdomen.
"All his tests have so far been normal, so we will have to wait and see what these new tests show up, but we're not expecting anything significantly different from what we already know.
"Effectively, the doctors think he has a post-viral fatigue syndrome after the shingles, which is a vague diagnosis.
"The results of the latest tests and ultrasound will be discussed, but the doctors have suggested Owen has a complete rest. They have recommended he has a break from the training ground and the football environment.
"It's impossible to say how long he will be out for, but it could take several weeks before he has fully recovered."
A knee injury has also caused the 18-year-old problems during pre-season.
Meanwhile, former Blues Darren Bent and Darren Ambrose have both been included in the England U21 squad for their game against Moldova at Portman Road next Tuesday. No current Town player has been named in Peter Taylor's party.