"Turnovers," St. Louis coach Craig Berube said. "The second period, I didn't think we skated very well. Also, penalties were a part of it all, but the second period we got pucks and we didn't skate, didn't advance them, turned them over, gave them momentum."
The Blues put the Bruins on the power play twice in the last 6:45 of the first period and twice more in the first 11:04 of the second. They finally connected on McAvoy's goal, but killing that many penalties affected the Blues' flow and their ability to roll four lines.
Boston, which entered leading the Stanley Cup Playoffs on the power play (34.0 percent), was 1-for-5 on the man-advantage.
"It takes a lot of guys out of the game and that burns up a lot of energy from other guys that are killing all the time," Berube said. "It's too much. We've got to be better there. We've got to be more disciplined."