I have an inside source that has told me Dorion never intended to re-sign Debrincat, the source is called mathemetics.
Dorion offered Tarasenko (Debrincat's replacement) 5.5M long term. Pinto was apparently offered 2Mx2. So in some sort of alternate universe where Tarasenko either accepted that long term deal, or sings one in January, that gives the Senators only about 6 million of cap space with a 17 player roster. Where is the other 3M+ for Debrincat coming from?
You have to keep in mind that most NHL teams project forward. So they knew at the start of last year that if Sanderson was what they though the was, he would cost 5M-6M on a bridge or 8M-9M long term. The Senators haven't had any surprise breakouts who forced their hand cap wise. Everything basically went according to plan. So it doesn't make sense that they would somehow peg Debrincat in at 8M-9M, which is what he'd likely take in Ottawa.
Although, based on how Dorion has operated so far, maybe the plan was to play with a 7 skater roster.