For me you deal from strength and at the appropriate time and that time could very well be this summer.
lets take a quick look at what they could have next year without Krug in the line-up. lets say going into camp next year this is their top six with J.Moore as their #7
Chara McAvoy
Grizz Carlo
Clifton miller
After that you have guys like Vaak and Lauzon, who too me are NHL ready right now, all they need to do is stay healthy and on top of that they have Zboril, who I don't think is too far away, add in the addition of Zech who will be entering camp and all of them will be fighting for a jobs. Just me, but I think that's pretty deep not just quantity but some real nice quality as well.
That's a pretty deep group , and without Krug and like I said after next year he hits RFA status and will be looking for a substantial pay raise. The option will diffidently be there to trade Krug if the right deal becomes available and add to that Sweeney greatest strength is his ability to find and draft NHL caliber defenseman. Its a great asset to have as there are always teams looking for defenseman and right now Sweeney has shown he has the ability to find them, in the draft or FA.
It would be fool hardy not take advantage of Sweeney eye, for acquiring defenseman, maybe it won't be Krug and they'll choose to keep him and deal someone else but there' more glaring holes to fill out-there, Like a future #1 Center or a true shutdown left-side d-man and dealing from a position of strength is the best way to address it. Krug is a tremendous offensive talent but you have to give to get, and too me the Bruins would be dealing from a position of strength.