Based on Sweeney's favorable experience breaking in as Ray Bourque's D partner, then watching Bourque break in Kyle McLaren and later Hal Gill, and most-recently Chara breaking in Dougie, coupled with Zach Trotman's development this past season in Providence (and his one-way, cheap deal for 15-16), I will be real surprised if Trotman does not skate right side with Chara.
Krug and McQuaid have been such a solid pair that I expect the only dynamic scenario is on second pairing, where Dennis Seidenberg skated last season with mostly Matt Bartkowski, a little with Joe Morrow and some with McQuaid. Add Colin Miller to this mix, a 22-yo PMD who established himself as a power skater and big shooter (won the lap and set a 105 mph mark with slapshot in AHL All-Star skills), just won the Calder Cup with Manchester ... he and Morrow (who is at the same career stage) are in the most-direct battle of camp for a spot on the NHL roster. A recovered Kevan Miller can play a hard-nosed third-pairing game with Krug if McQuaid has to kick up.
But, while Seidenberg is the only left shot regularly deployed to the right side and Krug can cross over in a pinch (Morrow also did it in the AHL but not without some hiccups), none of the right shooters have established themselves as either-side players, and Julien is pretty strict systems-wise on this one thing: the rights play on the right and the lefts play on the left.
We can't pretend it's 1990 and expect that some right shots would be slid to the other side or vice versa. Julien's list of switch hitters is short. Mark Stuart and Andrew Ference did it when the body count demanded it, but Seidenberg is the only lingering player that Julien deploys like he has Patrice Bergeron at center (ie. doesn't matter who he plays with or on what line because he is implicitly trusted with the role and the assignment). Now whether those days are behind Seids in his first full season post-surgery is something yet to unfold. If Don Sweeney has an opinion on the matter to the negative, then that would be the motivating factor in any roster changes to the defense.
He gives every indication that Trotman, Morrow and Colin Miller are ready to continue their development in the NHL, and all the signs point toward Trotman at least getting an audition to pair with Z (as did Hamilton in the pattern alluded to in prior era including Sweeney's).
Chara-L ... Trotman-R
EITHER: Morrow-L ... Seidenberg-L
OR: Seidenberg-L ... C. Miller-R
47 Krug-L ... 54 McQuaid-R
7th: K. Miller-R
Krug, a great competitor who understandably aspires to a top-four role (presumably as Seidenberg's left-sider but it could actually run opposite with them), always gets his minutes via power play. But I don't expect the Bruins to operate with a great luxury of experiment deploying him against top-six forwards on a regular, 5-on-5 basis. It's not about toughness or even size -- Krug holds his own in the containment game -- it's his stubby legs and vulnerability to transition attack (ie. lack of raw, open-ice speed). It's a part of his game that improved last season, but I don't think he's marginalized it to the point that the better players won't burn him. He's agile like few but that's a lingering ceiling on his career.
for "yes" and 
for "no".
