Bolton defender Gary Cahill could miss the rest of the season after suffering a blood clot in his arm.
It is a devastating blow for Cahill, who was hoping to claim a place in England's World Cup squad.
The 24-year-old was taken ill on Friday and sat out the 0-0 draw against Fulham on Saturday.
Manager Owen Coyle said: "Gary suffered a pain in his arm and it became severely swollen on the day of the game.
"There was no way he could play so we took him straight off to hospital. It turns out he had a blood clot in his arm, so he is going to be out for a number of months."
Cahill was due to be released from hospital on Monday after being kept in since Saturday.
Coyle continued: "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.
"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."