If you would prefer Grabner over Nash I don’t know how much you have watched. Assuming you didn’t watch the Rangers a ton, Nash is our most dominant forward night in and out in all three zones. Where as Grabner is good at one thing which is getting breakaways. Now you may have watched a lot I guess and prefer Grabner, but Nash is by far the better player in most Ranger fan opinions.
Nash solves the Krejci line's biggest issue, their defensive game, but especially their ability to clear the puck by winning puck battles along the boards and making good outlet passes. DeBrusk/Krejci/Spooner got OWNED by the Leafs last night. Adding Nash to the line gives it the same thing adding Heinen to that line does, a smart two way winger who can get the puck out of the zone. Heinen/Krejci/Backes has been the most effective grouping of that line. Unfortunately, DeBrusk/Riley Nash(defensive specialist/Spooner(offensive specialist) is not a line that makes sense.
Spooner has been an offensive force on Krejci's wing, but he sucks winning the board battles needed to clear the zone. DeBrusk is a rookie and makes rookie mistakes. Nothing to be concerned about longterm with DeBrusk, just being a rookie. You can get away with one of them on that line, but both is hard. Now, you are giving up some offense here. Spooner to Nash, at this point in their careers, is an offensive downgrade. Nash isn't the totally ideal rental, but better than the rest for Boston's need on that second line.
The return is the question here. The Bruins have to lose some cap in the deal. If they can be moving out Beleksey, that has some real long term pros and they should be willing to pay for that. I'd probably do 1st + one of the good prospects + Beleksey for Nash. Rangers fans will ask for another asset, and I would say "I would never give 1st + one of the good prospects for Nash...you already got the extra asset, come back crawling on Monday". I suspect Sweeney would say the same as the market for Nash is very small. You could also move out McQuaid and/or Spooner for futures to try to get back some of what you gave up for Nash in a different deal if you can't include Beleksey, but that is less compelling. LOTS of moving parts to make it a deal that is a good one for the Bruins.