"Karl is going to be out of action for a few weeks," McCarthy told the club's official website.
"He was kneed in the ribs and scans have shown that he has a bruised spleen.
"He's lucky because it could have ruptured. It's a good job that we have these sort of scans so that we know what's wrong.
"He will have to rest until another scan shows that it has cleared up.
"We have no history of these kind of injuries so it's difficult to know how long it will take. But I can tell you that he won't play whilst there is any risk to him."