The 20-year-old has featured in Paul Lambert's side four times this season as back-up to first-choice keeper John Ruddy.
Teenager Jed Steer was brought in as reserve keeper for Saturday's 2-1 win at West Brom as a result of Rudd's injury.
Lambert confirmed Rudd is set to undergo surgery which will rule him out for up to two months.
"Declan's hurt his ankle and it looks like he's going to be out for at least a couple of months," said Lambert.
The injury could force Lambert into the transfer market for a new goalkeeper and regarding any possible transfer activity in January, Lambert added: "I'll have to sit with my coaching team and see what we're going to do. I'll wait and see."