Byrne, 29, has not played since January due to the problem but had returned to full training and was lined up for a return to the reserve team this month.
That has not happened, with the club looking into the possibility of further nerve damage.
"We have sent him for a test on his nerves, to ascertain whether or not they are all functioning properly," manager Alan Knill told the club's official website.
"I am gutted for him because he came back and joined in training and everything appeared to be going well. Then all of a sudden it's not great
"He has missed 10 months with an injury which at most should have been three or four."