FWIW, Torey Krug and Matt Grzelcyk both have solid advanced stats, with the notable edge going to Krug. So, while the eye test may tell us that Grzelcyk is stronger defensively than Krug, there is no data, aside from +/- to back this up. Neither player is an obvious choice as a defensive top 4D. But both drive positive possession stats when they are on the ice, so I am not convinced that Krug is as bad as a few of you are saying nor am I convinced that Grzelcyk would be better than Krug defensively, if you gave him top 4 minutes.
Also, while there are notable cases of Krug coughing up the puck at inopportune times, this seems to be an anomaly for the 2017-18 season. Considering his TOI, he is relatively reliable with the puck. He isn't even in the top 100 players for "giveaways" and several name brand top players give up the puck a lot more than they cause turnovers the other way. It's a weird stat.
The reality is that the Bruins are using Krug the way he should be used (70% o-zone start vs. 30% d-zone start) and they are winning because of it. Not only is Krug racking up points, but the players skating with him have an 11% shooting percentage for the season. Why? Most likely because Krug is another scoring threat and defenders need to keep him in check at all times. Replace him with Grzelcyk and that shooting percentage would likely drop over time, since he doesn't shoot as often. That is an assumption, but I think it is accurate.
So, folks who propose trading Krug for a forward should keep in mind that taking Krug out of the equation will actually weaken the team in the O-zone. Also, since his possession numbers are very positive, there is little evidence that they will be any stronger replacing Krug with Grzelcyk.
If the Bruins were to trade Krug for an upgrade at LHD - essentially replacing Krug's own spot - they should look for a two-way defender who can skate, shoot the puck and defend. Teams don't generally part with those pieces...but that is a possible move.
Lastly, whoever it was that commented on the Bruins/Pitt game and how Mike Sullivan was rushing to send out their top dogs whenever Krug was on the ice...
Krug finished with a +1 and 17 mins played
Grzelcyk finished with a team low -2 and 16 mins played