Martinez said: "We don't need to sell him and we want him to be an important player for us next season."
The Spaniard feels that Scotland gets better with age and hopes that Swansea can fulfill the Trinidian's desire to play Premier League football.
"Jason is a late developer. I don't think he is the same as other players at his age.
"His best years are still in front of him. He is not like a normal 30-year-old where, having started at 18, his body is damaged.
"Jason is exactly the opposite and his experience is helping his talent to be effective on the pitch. So the older he gets, the more experienced and clinical he will be and he still has many, many years in front of him."