Cruising in the high slot, MacArthur one-timed Drew Stafford's pass from the left boards past goalie Tim Thomas for the win.
Ales Kotalik also scored for Buffalo, which snapped a three-game losing streak. The struggling Sabres have now won three times in their last nine games after winning the President's Trophy last season.
Marc Savard scored for the Bruins, who lost for the third straight time. Boston has lost eight of its last nine in Buffalo.
Thomas finished with 27 saves and Marc Savard scored for the Bruins, who lost for the third straight time. Boston has lost eight of its last nine in Buffalo.
The Bruins grabbed a 1-0 lead on Savard's third goal of the season at 8:07 of the first period. From the left circle, Savard was able to convert on a rebound after Miller made the initial save on Glen Murray.
The Sabres tied it with 4:10 remaining in the second period when Kotalik scored on the power play. After accepting a pass from Maxim Afinogenov, Kotalik ripped a slap shot from the left point for his third of the season.