With the numbers MacArthur put up in bantam hockey, he was able to secure a tryout with the Medicine Hat Tigers, but was the club's final cut heading into the 2001–02 season. As a result, MacArthur played in the Alberta Junior Hockey League (AJHL) for the Drayton Valley Thunder. In his only season in the AJHL, MacArthur recorded 62 points in 62 games, and helped the Thunder capture a League championship. During the season, he grew seven inches and Medicine Hat was again interested in obtaining his services. He subsequently joined the club for the 2002–03 season, putting up 75 points in 70 games as a rookie. During the summer, MacArthur was then drafted by the Buffalo Sabres in the third round, 74th overall, of the 2003 NHL Entry Draft.