The popular left back went to Huish Park at the beginning of September for in an intial one month deal but settled in so well that Glovers' manager Gary Johnson decided to keep him for a further four weeks.
However Bull hobbled off after injury his knee during a game against Cheltenham Town and has returned to The Den for treatment.
In other news, defender Mark Phillips is expected to miss the rest of the season after injuring his cruciate ligaments.