The Bruins built a 3-1 lead in the first period and led by one following 20 minutes, but New York tied the game on a shorthanded tally with 8:55 remaining in the third. The Rangers then won it in overtime on a blunder by Linus Ullmark, who took a long pass from Pastrnak during the 3-on-3 session and tried to force a pass back up the wall.
Rangers forward Alex Lafreniere stepped in front of the attempt and sneaked a backhander by Ullmark for the win.
"He just made a mistake," said Cassidy. "Their player made a good play to not give up on it. If he gets it by that player, obviously, it's a 3-on-2. I'd have to look at it closer to see where everyone else was, but you expect him to move it past or hold onto it if they're in a change to make a play to one of our guys coming back.
"You want to keep puck possession in that situation to not let them change, so maybe Pasta, if he revisits it, would skate it back himself and wait for another outlet and then change."
Overall, Ullmark made 15 saves on 19 shots in his first full game action of the preseason. The newly signed netminder also made 11 stops on 13 shots in New York on Tuesday night in his exhibition debut.
"Well, it needs work," Cassidy said of Ullmark's game. "Little bit of a tougher game to play tonight. I thought territorially, we were good…the first goal, kind of a bang-bang play; our center got caught a little high, we had three high there, not able to recover. So, a little bit of a fortunate bounce, breakaway.
"All in all, it's preseason for him as well. Right now, I'm sure he'd be the first to tell you, he needs some more reps, and he'll get more reps, and we'll see where he's at after the Washington game [on Wednesday]."