The Blackburn boss said: "I had a meeting with David's agent two weeks ago and at that meeting I expressed the view that if the right offer came in for him we would listen.
"If he left here we would be reluctant sellers. But there sometimes comes a time in a player's career when he needs to hear a different voice, and for whatever reason we don't think David has been at his best this season.
"He's been disappointing this year. He's not come anywhere near the standards he has set himself. Maybe in David's case there are other things going on his life."