"Right now, we don't know much, but he won't be part of camp," Dorion told reporters without confirming that the issue is related to concussions.
MacArthur played only four games in each of the past two regular seasons, but his return to the playoffs last season was a feel-good story. He scored nine points in 19 games as the Senators advanced to the Eastern Conference finals. After the run, MacArthur revealed that he was dealing with a neck issue.
The 32-year-old was a solid contributor when he was on the ice before concussions started to pile up. He had 24 goals and 51 points in 79 games with Ottawa in 2013-14.
MacArthur is a three-time 20-goal scorer in his 11-year NHL career.
He has three years left on his contract with an annual average value of $4.65 million.
The Associated Press contributed to this report.