The enigmatic France winger has yet to justify his £9.5million fee since a summer move from the DW Stadium.
But Martinez said: "Charles is enjoying the experience at Villa and his football will get better.
"He is a top, top player.
"But it takes time to settle, especially for a flair player.
"It's not like being a centre-half, where you can just slot into the team.
"He hasn't been himself, but I'm sure that Alex McLeish will find a solution and he'll be a top player for Aston Villa."