The 24-year-old, who was being tipped as a surprise inclusion in Fabio
Capello’s squad for South Africa, was taken ill on Friday and missed Saturday’s 0-0 draw with Fulham.
Cahill was only released yesterday and has been told he may now miss the rest of the season.
Bolton manager Owen Coyle said: “It is a severe blow for the boy but our
main concern is his welfare. We have to make sure Gary is fit and well.
“He will miss the next few months, it might be for the rest of the season, we will hear from the specialist later.
“The clot needed to be resolved and in doing that you need the medication. It follows you can’t play. I don’t think it is a sport-related injury, I think it is just one of those things that can happen to anyone at any given time.”
Cahill was part of England’s squad for the World Cup qualifier against Kazakhstan in June but is yet to win a senior cap.