The Cobblers were undone by goals from Jason Lee and a Chris Willmott own goal as they went down against the Conference North side to suffer their first pre-season loss.
"As a unit we were poor without the ball," Sampson told the club's official website. "We were neat and tidy when we had the ball without having a cutting edge.
"The good thing is that it is pre-season and no points are won or lost. We won't get too down about the result but you can learn about players in these pre-season games.
"The most important thing is being ready for August 8."