The Bruins need some offense from the blue line while McAvoy and Grz are out early in the year. Barrie can certainly provide that. Skating with Forbort, his lack of defensive ability won't be as exposed alongside a stay-at-home shutdown D. On the flip side, Reilly is another LHD and showed last year that offense is not really his game but is a far superior defensive player to Barrie, which Edmonton needs. Lindholm, Grz, Forbort, Zboril, and Ahcan make him expendable on the left side. His health, lower cap hit, and more balanced game make him a more attractive trade candidate than Grzelcyk as well.
In a vacuum, I would trade Reilly for Barrie straight up if I was Boston and certainly if I was Edmonton (who have quietly been rumored to be shopping him for a while.) However, the cap constraints make it a bad move for the B's. Yet, if they can add Klefbom's contract to add to LTIR, which they will have to be in from the get-go due to a multitude of injuries to major players, the Bruins can then stay in LTIR all year and exceed the cap by at least Klefbom's $4.1MM all season. This helps to accommodate the cap increase of Reilly to Barrie while also making room for Bergeron, Krejci, and a surprise signing in Phil Kessel, all on 35+ bonus deals as follows:
Bergeron/Krejci: $2.25MM base + $2.75MM bonus
Kessel: $1.25MM base + $2.25MM bonus
The B's can stay within their LTIR window with this squad, Kessel can play on the top line with Marchand out and then plug into different roles to boost scoring as needed. He and Barrie will make the PP2 almost as dangerous as the PP1 and should thrive under Montgomery. This makes the Bruins dangerous on all fronts and makes them a contender once again, albeit only for one year.