Cahill season ended by clot
The 24-year-old had felt ill last week and was taken to hospital on Saturday, missing the goalless draw with Fulham, the club said on their website on Monday.
"He came in on Saturday and the swelling was quite serious and the medical staff decided to take him to hospital right away," said manager Owen Coyle.
"It turns out he had a blood clot in his arm so he is going to be out for a number of months...it might be for the rest of the season; we will hear from the specialist later.
"I believe he would have made the World Cup for the summer but we have to make sure first of all he recovers from this blood clot. Gary will come back and continue to be a top, top footballer."