MacArthur, 30, has played only four games this season due to concussion-related symptoms. The team said he has been cleared medically to play, but management and doctors decided it was best to play it safe and have him return next season. He finishes with no points this season after posting 16 goals and 20 assists last year.
Phaneuf has missed the past two games with an oblique injury and soreness in his right foot that started after he blocked a shot March 12. X-rays were initially negative on the foot, but further testing revealed a hairline fracture, the team said.
The 30-year-old Phaneuf was acquired at the trade deadline from the Toronto Maple Leafs, with the hope that he'd give Ottawa's playoff hopes a boost. But they have hovered around .500 since then.
Phaneuf finishes the season with three goals and 21 assists.
The concussion MacArthur suffered during the preseason wasn't his first. He also suffered one in February 2015, but the latest one was more debilitating, in part he thinks because he ignored the symptoms and tried to play.
"I feel like if I had just went with my gut instinct from day one, I would have missed 10 games and I would have played the rest of the year," MacArthur said to the Ottawa Citizen last week. "It would have been a different situation. It could have been a completely different year."
He tried to play even though he knew something was off.
"I would get a couple of passes and think a guy was on me and I would try to move it, but I had, like, 10 feet of room," MacArthur said, according to the Citizen. "I was stumbling around and I think my balance got tweaked a little bit. How do I bring it up that I'm not good now? I already told them that I was. I had to figure it out before everyone catches on that I can't even play."
He decided he had to come clean about his condition, but before he could he took a spill in a game Oct. 14 and hit his head. What followed were intolerable headaches, trips to specialists, depression and the realization that his career might be over.
A nerve block -- a long needle inserted into the nerve above his eye -- relieved some of the headaches. Slowly but surely he got back on the ice, only to experience vertigo. His return was delayed again.
Finally, by mid-February he was back practicing. The Senators had acquired Phaneuf and appeared ready to make a charge for a wild-card spot in the East.
But MacArthur didn't pass his baseline concussion test. Phaneuf got hurt. The Senators faded and decided to shut them both down and get ready for next year.