Summer signing from Shrewsbury Town Hall tore his calf muscle on the opening day of the season at Bury, only four minutes into his debut.
The tear reached as far as his Achilles tendon, which is a notoriously slow area to heal. After several weeks of intensive rehabilitation, Hall was given the green light by a specialist to step up his training in November, but another tendon has attached itself to his Achilles, causing considerable discomfort when he runs.
"This is an unusual complication and Asa needs keyhole surgery to correct it," senior sports therapist Ian Weston said.
"The surgeon needs to strip the tendon away from the Achilles and the rehabilitation from there should be straightforward."
Meanwhile, skipper Matt Taylor suffered a grade one hamstring tear at Exeter City on Boxing Day and is only expected to be sidelined for a fortnight.
Joe Hanks should be back in contention for the trip to Oxford United on Saturday as he recovers from mild concussion and a bleed on his eyeball, following an accidental collision with team mate Lee Vaughan, also at Exeter.
Player coach Steve Elliott continues to recover from bone bruising to his knee. The problem is improving and he completed a running session on Saturday, but it is too early to say when the big defender will be back in action.