An important job of a coach is to know that athletes -- the athletes you want -- are competitive to a fault: if you ask a gamer if he can play, even if he is standing there like this guy below, he'll still say, 'put me in.'
Monty has to make the call there. Managerial failure, if that is what happened.
If I dare say, I believe this wisdom also applies to Bergeron and Krejci in this series. Maybe it's a bit of a wild extrapolation, but I don't think it's a coincidence that the Bruins' best games in this series were the games where those two were out: 52% xGF and 4-2 final in Game 3, 62% xGF and 6-2 actual goals in Game 4.
Now this is pure hindsight, but maybe don't mess with something that's working? Coyle and Zacha stepped up big as 1C and 2C, and indeed
got better in their second game in those roles. Instead, Montgomery injects an injured Bergeron into the mix, and then decides to run the lines through the blender for... reasons?
I get that this year was viewed as the last ride of Bergeron and Krejci and there's rightfully a lot of sentimentality was wrapped up in that. And Game 5 was a home game which made for a convenient "triumphant return", but the Bruins were in good shape and had dominated the previous game. Now maybe Coyle and Zacha don't keep up their great play, but then you break the glass for the emergency Bergeron if the Bs drop Game 5. It certainly seems like Bergeron and Krejci were granted spots without a thorough examination of how their injuries would impact their competitiveness.
Now this is just going off vibes, but Montgomery strikes me as a "players' coach" because he goes along with his players and might be a bit conflict-averse with them. But as you say, being a head coach means having to make some tough calls, and the players are not always the best judges of their ability. This team was extraordinarily deep as Games 3 and 4 proved, and mistrusting that depth is an absolutely mystifying decision. Especially to what we're hearing with regards to Ullmark and Swayman.