The Irishman has not featured for the Wearside club since being substituted in November's defeat at Hull City after suffering a concussion in a challenge with compatriot Paul McShane.
Vito Mannone has usurped him as first choice at the Stadium of Light, and Westwood has since sustained a shoulder issue, which requires surgery.
However, he has been unable to go under the knife until due to illness, with manager Gus Poyet not expecting him back before the end of the season.
The Black Cats boss told reporters: "It [surgery] has been postponed because of a little illness. You cannot have an operation when you are ill, so he is on antibiotics now.
"I would like to say [he will be out for] three to four months.
"Now, if you are going to think when is the season going to finish, it's probably to the end, yes."