Sens fan here intruding.
Reilly has everything to be succesful and really got much much better as the year went on. He's a guy who always had it in him back to his Minny days but never established himself or never really had the chance to play and make mistakes. Something we often see with tweeners or young players, they get limited opprtunity and when they make mistakes they're out of the lineup.
This year, considering how bad the Sens D was, he got the chance to play in meaningful minutes and wasn't benched for every little mistake. He got the chance to stay in the lineup and work on his game and really refine his defensive side. By the end here in Ottawa, he was really aggressive and physical all over the ice, breaking plays and like you mention he has good skating and vision.
If he can be paired with a guy that he can lean on defensively I think he will flourish and could be an underrated move for Boston, even past this season.
Pretty much every Sens fan hated Reilly to start the year but as it went on it was undeniable that he got much better and got some praise from DJ Smith and the fans really warmed up to him. I know some fans wanted him to stay but with our left side pretty much locked in as of next season it didn't make sense to keep him.
Hoping for the best, by all accounts he was a really well liked guy in the room and by our coaching staff.