The 31-year-old MacArthur hasn't played since Oct. 14, 2015, because of post-concussion syndrome. He suffered another concussion at training camp this past September.
Dorion told local reporters Friday morning that team doctors advised them to shut down MacArthur for the season. MacArthur still hopes to return next season.
MacArthur is a three-time 20-goal scorer in his 11-year NHL career and scored a career-high 24 goals in 2013-14, his first season with the Senators.
In the wake of the news about MacArthur, Dorion said that he would strive to make a trade for a forward before the March 1 trade deadline.