Midfielder Montgomery returned to the starting line-up for the midweek win over Nottingham Forest but is struggling with an on-going knee problem while on-loan Wolves striker Vokes has been nursing a hamstring injury in recent weeks.
Another on-loan player, Bjorn Helge Riise from Fulham, will not be in contention as he has had an injection to aid his recovery from a hamstring strain. However, striker Ched Evans, who missed the win over Forest due to a knee injury, could return to training on Friday in time to be in contention.
The Blades secured their first win in 15 league matches - and manager Micky Adams' first in charge - against Forest.
Adams has no other new injury or suspension problems but Chris Morgan, Andy Taylor, Ryan France, Richard Wright and Darius Henderson are still out.
Source: PA
Source: PA