Losing Muzz hurts a lot. He's our best, most well-rounded defenseman in my opinion.
It hurts less though in this series. Given that we are playing a team that has a mindset of 'protecting a 0-0 lead', there isn't really a top scoring unit for us to match Muzzin/Holl against. Not that Atkinson, Dubois, Bjorkstrand, & Nyquist don't pose an offensive threat...but Tortorella isn't exactly focusing on using them for offense, as much as balancing them across his lineup to create effective shutdown units.
As far as Marincin vs Sandin goes...I'm a fan of Marincin for this role. He's not a good player, but people are very hyperbolic about how bad he is, and its clear to me that they don't watch hockey very well or closely. He's not really good, but neither would I say he's 'bad'...he's pretty much replacement level, quite neutral, doesn't add anything, but doesn't hurt to play as a 6D.
Yes, he handles the puck like a live grenade, but he does so many good, subtle positional things defensively. In terms of role he fits the mark as filling in Muzzin's PK shoes (which Sandin cannot do), and also replaces Dermott as a more defensively reliable partner for Barrie. Sandin + Barrie was a bad combination this season, not complimentary styles at all, basically got caved in. In contrast, Marincin + Barrie was quite good as a 3rd pairing (somebody earlier posted their statistics, as well as Dermott/Holl statistics).