One defenseman doesn't make your entire forward group start playing better
I mean, to some extent it can...especially when that defenseman is playing almost half the game. The forwards aren't exactly on the top of their game right now, especially relative to the Cup year, but they're receiving a fraction of the support they received from the blue line that year and it's a lot harder for them to have that sort of impact without that support. Let's look at some of the ways they're fighting an uphill battle.
1) Better defense leads to less time in the defensive zone. That means more less opportunity for your forwards to be exposed defensively, and less energy wasted defending.
2) Better defense leads to better transition play. Again, fewer turnovers means less defensive responsibility and energy wasted. On top of that, entering the offensive zone with speed, and even more importantly control, is a huge facilitator for offensive play. Springing odd-man breaks is an added bonus.
3) A better defender can lead to more opportunities in the offensive zone, whether that comes from rebounds on good, hard, accurate point shots, or good shots off seam passes, or breaking down the defense at the line with some fancy skating/stickhandling, or presenting weak-side attack options, etc. More threats/options in the offensive zone makes it easier for the forwards to create offense.
I'm not saying that adding Pietrangelo would magically turn this series around. There's way too many other problems and complicating factors to make a statement like that. But one really talented 3 zone defenseman can cover a lot of warts and make it a whole lot easier for his teammates to get their jobs done, and in that regard, this team
is being affected by Pietrangelo's absence right now. I'm not offering that as an excuse, or to start a tangent discussion about this past offseason. IMO, it's just a fact, and one that should be recognized...not so we can second guess the past, but so we can recognize what issues we face and that must be addressed as we move forward.