Ottawa Senators forward Clarke MacArthur was diagnosed with a concussion Sunday after taking a hit from defenceman Patrick Sieloff during a training camp scrimmage.
MacArthur, who also dealt with a concussion 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.
Sieloff was acquired from the Calgary Flames in exchange for Alex Chiasson this past June and was looking to make an impression with the organization.
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," Boucher said. "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 premeditated, it's 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."
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 postconcussion syndrome. He had been training since March, when he was given medical clearance.
The Senators play their first preseason game Monday night in Halifax against the Toronto Maple Leafs.