MacArthur, who also dealt with a conussion last year, was hit at the boards and dropped to the ice.
General manager Pierre Dorion said MacArthur will be evaluated daily.
“We don’t know much, this just happened an hour ago,” Dorion said. “He spent time with our doctors and medical staff and we sent him home and he will be re-evaluated Monday.
“I can’t give you too many answers. We just felt that Clarke is such a good guy, such a big part of our team that I should speak and try to update you as much as we can.”
The incident stunned the crowd on hand at Canadian Tire Centre for the organization’s annual Fan Fest.
Bobby Ryan immediately went after Sieloff and dropped the gloves. Play resumed shortly after, but Chris Neil looked to settle the score on Sieloff’s next shift. Sieloff was removed from the game as a precaution.
Head coach Guy Boucher said he did not feel the hit was intentional.
“When you’re sitting in the stands you see lots of space, but that’s not what it’s like on the ice,” said Boucher. “When you’re a player all you see is bodies in front of you, one in front of the other. Things are happening in a fraction of a second.
“I can guarantee you that if you ask him who was in the corner he has no idea who it was. We were in the stands and I wasn’t even sure who it was. When you can see him it’s because it was pre-meditated, its because it’s retaliation and a player is going after someone, but when it happens quickly during a game you don’t see who it is. Whether it’s in society or in hockey you have to be very careful before laying blame, before pointing fingers.
“There was nothing pre-meditated, nothing exaggerated, but it’s sad because it happened to someone that’s important in the room and someone we wish well because of what happened last year.”
The 31-year-old MacArthur was looking to make a comeback following a difficult 2015-16 season that saw him miss all but four games due to post-concussion syndrome. He had been training since March, when he was given medical clearance.
“The first thing we need to think about is Clarke the human being,” said Dorion. “We saw everything Clarke went through last year and we see what happened today and we think about the human being and his family, his kids. That’s the most important part for us right now.”
Players struggled to talk about what took place and what lies ahead for MacArthur.
“First and foremost I’m worried about him, his family and his well-being,” said teammate and close friend Dion Phaneuf. “I believe everyone just from being in the room with the guys that everyone is very concerned for him and we’re thinking about him, we’re thinking about his family and we’re hoping that it comes around for him.
“That’s all that we’re able to do right now, but it’s a tough day for everyone in our organization knowing what he went through and then seeing that (Sunday).”
The Senators play their first pre-season game Monday night in Halifax against the Toronto Maple Leafs.