Johnson signed a new one-year deal with the club on Wednesday, with an option for the 23-year-old defender to remain at Sixfields for a further 12 months.
While Boothroyd is delighted that one of the key members of his team from last season has decided to stay, he also made it clear that he expects plenty more from a player that came through the academy system at Middlesbrough.
"We saw what John was capable of at times last season, but he wasn't quite consistent enough and achieving that consistency is the challenge for him moving forward," said Boothroyd.
"I know he will work very hard and rise to that challenge and I am looking forward to helping him realise his potential and become an even better player.
"Like all of the players, John has been given a fitness programme to follow over the summer and that will ensure he will be back for pre-season in tip top condition."
