Buttner has begun training with United for the first time today.
Van Stijn told Algemeen Dagblad: "Every season the club calls all their European scouts for a list of players.
"They ask you for a top five of the best players from the Eredivisie in any position. On the list, at the end of last season, I had Buttner at number three or four.
"I would place him behind Jetro Willems (PSV)."