The 19-year-old scored eight goals this season including one in the League Cup third-round victory over Liverpool back in September.
His performances, which earned him the club's Player of the Year gong, are likely to have attracted the interest of a number of clubs but Johnson believes it is in the player's best interests to stay at Sixfields for the time being.
He told the Northampton Chronicle and Echo: "He's going to be a top player. (But) I think he probably needs another year or so with me and then he'd be ready for a move. He needs to be playing.
"There are still things he needs to learn and I want to help him do that so hopefully he'll be here for a year or two yet. You can build a team around players like that."
Source: PA
Source: PA