During a training camp game on September 25, 2016, MacArthur suffered another concussion when newly-acquired Patrick Sieloff hit MacArthur into the boards. Bobby Ryan immediately began a fight with Sieloff, which was quickly broken up. Sieloff was sent to the Senators' AHL affiliate the Binghamton Senators shortly afterwards. On January 20, 2017, the Senators announced that MacArthur would not play any of the remaining games of the season on the recommendation of various doctors, despite MacArthur's insistence, that he did not have any symptoms of post-concussion syndrome. On April 4, 2017, MacArthur returned to the Senators' lineup after an 18-month absence and played in all of the Senators' four remaining regular season games. On April 23, 2017, MacArthur scored the series-winning goal (on the powerplay for a 3-2 win 6:30 into overtime of Game 6) against the Boston Bruins in their first round playoff matchup.