The 26-year-old, signed permanently on a free transfer from Blackpool in the summer after an initial loan spell last season, was originally expected to be sidelined for a month after pulling a muscle in his thigh in last weekend's win at Leyton Orient, but scans have revealed the full extent of the injury.
"It's a lot longer unfortunately, it looks like it's going to be between eight and 12 weeks at the moment," said manager Danny Wilson. "He's got quite a severe thigh strain and at this moment in time anything before that will be a bonus, so we're a man down in that area and it's important we bring somebody in to supplement the goalkeeping area.
"I feel for Mark quite honestly because he was just coming on to his game as I said the other week. I thought he was really starting to put his mark down in the team.
"His performances were very relaxed in terms of his dominance and his relationship with the back four was getting better and I'm gutted for him because he was desperate to come and play for us last season, let alone this season and it's sad it's been curtailed a bit."
Wilson added he had been met with several "maybes" and "possibles" in his bid to draft in cover for Howard on loan and was not expecting a new arrival in time for Saturday's clash with Oldham, with teenager George Long set to continue in his absence.