McCarthy is pushing for a move in the transfer window and Blackburn boss Allardyce will consider a sale for the right price.
Allardyce said: "I think Benni has made it clear through his agent this week that he would like to see if there is anybody out there who would be wanting to take him and play him on a more regular basis than we are.
"As always when you have a player who is a little unhappy you will always look at the situation and makes sure that it suits you first, and if it does then it can get done. I understand the frustration and I have explained the situation not just to him but to everybody.
"David Dunn has been by far the most productive player in the team, he's basically stolen Benni's position and that's way it is at the moment.
"David has been doing fantastically well and he's been one of the main reasons why our home form has been that of a top-six side. It's a different kettle of fish away from home of course, where we look like a relegation side."