This would become the postseason's most unlikely setting for one of its grandest predictions.
As Blues forward Oskar Sundqvist relayed after the game, Berube told the team that he was using the bathroom after the third period when Gunnarsson walked up and stood next to him.
"And all Gunnarsson said to him was, 'I just need one more chance,'" Sundqvist said. "It was true, apparently. It worked out."
After ringing the puck off the post late in the third period, Gunnarsson scored the winning goal in Game 2 at 3 minutes, 51 seconds of overtime, draining a shot past Tuukka Rask from the blue line on a delayed penalty against Boston.
It was Gunnarsson's first playoff goal in his 57th career postseason game -- and just the third time in NHL history that a player's first playoff tally came in overtime of a Stanley Cup Final game.
"I don't score too many, but nothing comes even close to this," Gunnarsson said. "So it's a pretty good feeling right now."
Gunnarsson had every reason to make his powder room proclamation, because he had beaten Rask twice before at TD Garden this season. On Jan. 17, he blasted a shot from the slot past Rask in a Blues win, one of just three goals Gunnarsson scored in an injury-riddled season. On Wednesday night in Game 2, with 1:57 left in the third period of a 2-2 game, Gunnarsson beat Rask with a shot, but it hit the iron and trickled away.
"[I shot it] as hard as I could, yeah," Gunnarsson said of his third-period shot. "I knew I had it past him, but then I saw it sitting in the crease. I was hoping someone would poke it in, but that didn't happen.
"But luckily, I was able to score in the OT."
Blues defenseman Colton Parayko said Gunnarsson made that "one more chance" count.
"That was an absolutely howitzer," Parayko said of the winner. "He's an unbelievable defender. I've been here for four years, and he does it all. He's been outstanding."
Despite his success in Boston, Gunnarsson isn't exactly known for his offensive acumen.
"I liked hearing it," Berube said of Gunnarsson's talk before overtime. "He hit the post in the third there and he felt good about himself, obviously, which he should have. He had a hell of a game, I thought, and I'm really happy for him that he ended up getting that game winner."
It was the kind of moment the Blues searched for in their Game 1 loss to the Bruins, needing someone to make a play as St. Louis trailed by a goal deep into the third period. In Game 2, the Blues were a different team in a variety of ways -- controlling the game with their forechecking, keeping the Bruins' offense in check and getting high-danger scoring opportunities against Rask deep into the game.
But the Blues needed a play in overtime, and Gunnarsson provided it.
"First game, we weren't happy; today, we came out flying, and I think we played a great game for 60-plus minutes," Gunnarsson said. "That's just the team we are. We come back like that. We never give up. For me to put it in there and get the winner, it's a great feeling."
The goal gave the Blues their first Stanley Cup Final victory in franchise history. The series now moves to St. Louis, where a sold-out watch party was flush with anticipation for Game 3.
Gunnarsson's bathroom proclamation becomes the latest entry in Stanley Cup playoffs lore.
"You don't hear that story very often. That's not a place to have a conversation," Blues captain Alex Pietrangelo said. "But I guess it works."