"There was no way he could play so we took him straight off to hospital," said Bolton 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."
Everton step up search for recruits as Joleon Lescott nears £94k-a-week Man City moveJoleon Lescott faces medical in £24m move from Everton to Manchester CityHull City 1 Bolton Wanderers 0: match reportCahill was due to be released from hospital on Monday but the treatment for his condition will prevent him playing for the forseeable future.
"It is a severe blow for the boy but our main concern is his welfare," Coyle added. "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.
"As disappointed as we are, at 24, Gary has a marvellous career ahead of him. 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."
Cahill broke into Fabio Capello's England squad last year and had been widely tipped to make the squad for South Africa given the injury doubts covering a number of the defenders who are above him in the international pecking order.
Even if he could make it back in time for the end of the season, it is hard to see him meeting Capello's criteria for inclusion in his final squad, which are that players are playing regularly for their clubs and are fully match fit.
The loss of Cahill is also a severe blow to Bolton, who are battling for survival in the Premier League. Coyle's side travel to Manchester City on Tuesday with just one point keeping them out of the relegation zone.