Haji Mnoga is out for eight weeks - after injuring himself while walking to Victorious Festival.
The highly-regarded right-back had been pencilled in for first-team involvement at QPR in the Carabao Cup.
Yet instead he is occupying the treatment room after breaking the fifth metatarsal in his foot at the weekend.
According to Kenny Jackett, Mnoga twisted his ankle having fallen over a kerb.
The 17-year-old was heading to Victorious Festival, after representing the Academy in their 3-0 win over Oxford United on Saturday.
As a consequence, he has been left on crutches and is expected to be absent for two months.
Jackett had planned to name the youngster on his bench at the very least for the first-team’s trip to Loftus Road on Tuesday night.
In addition, there was the possibility of a right-back start against Crawley in next week’s Leasing.com Trophy opener.
Mnoga made his debut in the competition in October 2018 against the same opposition – now he must sit it out.
Jackett said: ‘Haji basically went over on his ankle and broke his fifth metatarsal.
‘He didn't do it in Saturday’s Academy game, he did it outside.
‘He was walking along the street and gone over the kerb on the way to Victorious, he didn’t make it.
‘It’s a freak injury and a metatarsal as well, which is very sensitive. It’s frustrating, but injuries happen.
‘Haji’s a young player and had a very good pre-season, in training and games he looked excellent. He’ll get over it.’
The former Trafalgar School pupil is the second-youngest player to represent the Blues in post-war history.
He was aged 16 years, five months and 24 days at the time of his debut in the 1-0 win at Crawley.
Jackett added: ‘There was the possibility for Haji in the Carabao Cup at QPR – and next week against Crawley.
‘He’s had a good pre-season and would have been at least on the bench and a consideration for right-back.
‘I would have considered him. James Bolton has been signed as a first-team player and suffered quite a bad tackle at Shrewsbury, struggling with it ever since.’