There's pros and cons with any selection choice on D. Play Gryz and you'll definitely have a better time generating offense, but then you've got less size and physical presence to work with in protecting the crease against a team that likes to muck around the net a hell of a lot, and one less option to use in killing penalties and late leads. That's especially relevant to me when Lindholm was clearly under the weather. Forbort's ability to protect the house and soak up tough minutes is really useful against a high-shot, pucks to the net team like the Panthers.
You could also consider Gryz over Cliffy but then you're removing a right D for a left, and that means moving more pieces around and potentially losing some cohesion. Still an option, but not a simple or self-evidentiary one.
I think Monty has at least three things in mind with scratching Gryz - there are other defenders who are more than capable of stepping up and delivering good transitional play (McAvoy, Lindy, Orlov, even Clifton) and they need to do so, more than they did last night; he wants defenders who are multi-situational and give him versatility in dealing with particular circumstances, and Forbort offers more of that than Gryz; and playoff games are won from the back end, so make sure your play in your own zone is as strong as possible and then take care of the rest, and again it's advantage Forbort here.
As I said in another post, I'm sure we'll see Grzelcyk at some point and I back him to do a solid job when he gets his chance. But for now, as he's proven a billion times by now, Monty knows what he's doing, so he has all my trust.