After starting the 2006–07 season in Rochester for a second year, MacArthur made his NHL debut for Buffalo on December 19, 2006, against the Montreal Canadiens. He split time between the Americans and Sabres for the rest of the season, finishing with 63 points over 51 games in the AHL and seven points over 19 games in the NHL. He recorded his first NHL point, an assist, on February 20, 2007, against the Philadelphia Flyers and scored his first career goal against the Ottawa Senators on February 22.