Jake DeBrusk batted in the rebound of a Pastrnak shot on the power play to put the Bruins ahead 2-1 at 9:15. Krejci got the second assist to reach 700 points.
It was DeBrusk's third goal of the season and second in four games.
"Happy he scored, obviously, because like I said, he is as scorer," Cassidy said. "We measure him by them. He really measures himself by them, obviously. So when he gets rewarded, it really fuels him. So that's good. Hopefully, he gets hot. We need it."
Craig Smith extended the lead to 3-1 at 18:36 when he scored from the slot after Krejci poked a loose puck to him.
"It's been incredible," Smith said of playing with Krejci. "Obviously, it felt good to get one in there, but [Krejci], he's a great player. He's able to slow it down, he's one of the smartest players I've played with as far as talking on the bench and little plays he sees out there."
Pastrnak scored on a one-timer from the left circle off a pass from Krejci on a 5-on-3 power play to make it 4-1 at 4:44 of the third period.
"I felt today, at least within our own game, we established at least a decent pace, and we didn't sustain it," Granato said. "I could say we couldn't sustain it, but the fact is we didn't sustain it. So now you look into that and say, 'OK, how can we sustain it?' It might be better puck management. Did we mismanage pucks tonight that cost us more energy in getting them back? When you mismanage a puck, you put yourself in a bad position and spend more energy to recover it, not feeling you were going to, obviously, misplay a puck. So they all tie in, but I do believe our guys played with, at least at the outset, a good pace in sustaining it again."
NOTES: Krejci is the eighth player in Bruins history to score 700 points. He has 494 assists, two behind Rick Middleton for seventh in Boston history (496). … Bruins forward Trent Frederic left the game at 18:38 of the first period with a lower-body injury after he was hit by a puck. Cassidy said X-rays were negative. ... The Sabres are 0-for-19 on the power play in their past 12 games.