And the latest setback has prompted the Blues to weigh up the possibility of surgery on the sidelined midfielder.
After recovering from injury, Morris travelled to Sunderland as 19th man for the 2-1 Stadium of Light defeat earlier this month.
However, last week the problem flared up once more.
Jackett believes the issue stems from the stomach area – and has no idea when the former Shrewsbury man will be available for selection.
He said: ‘Bryn had a bad week last week – and has a problem in the stomach area as well.
‘We are slightly mystified but, even so, need to look at it a bit deeper now, maybe it lends itself towards an operation somewhere.
‘He has a problem in the stomach area which he just can’t shake off, especially when checking, turning and striking balls.
'It’s just above the groin area in the pelvic and pubic areas, and they are sensitive areas.
‘We have given him a lot of rest, with different build-ups. This time last week we were thinking he was 100 per cent – now he’s actually struggling.
‘We do think we have given him enough time and through the summer as well. It has surprised us, we thought it would naturally heal itself through the summer.
‘We’ve had it scanned by a top professional and the top-two London surgeons in that field have received the results as well.
‘But we still haven’t been able to rectify it.’