The player wanted to leave in the summer, but snubbed Birmingham because of his wage demands.
Now the Mackems are after him, and Steve Bruce was in the crowd at StJames' Park on Saturday.
Roberto Martínez: "Last season Charles took his ability on to another level and this season he has carried on.
"He understands why he would get a bad reception, but he's got experience and, in a way, he sees it as a football compliment. I was glad to hear the boos.
It makes you feel like you're a threat, someone who can hurt the opposition."