N’Zogbia bagged a brace in the 2-2 draw at Newcastle on Saturday.
“Charles’ value will go up because he is only 24 and playing for the national team,” said Martinez.
“But he’s someone we don’t want to lose.”
He added: “Last season Charles took his ability to another level, and this season he has carried on. He’s improved a lot.
“He understands why he would get a bad reception here (Newcastle), but he’s got experience and he sees it as a compliment.
“It makes you feel like you’re a threat, someone who hurts the opposition.
“You could say I was happy to hear the boos. It drew a reaction from him. We knew how the Newcastle fans would react and he relished the challenge.”
