The sight of a limping Tuukka Rask practically soured the high of a 4-3 win over the Vegas Golden Knights on Tuesday night.
"He either had a little bit of a muscle pull or a little bit of dehydration or maybe a little of both", Cassidy said after the win, which gives the B's their best start to a year since 2001.
The goaltender briefly stayed down on the ice, was slow getting up and had to be helped off the ice. Max Pacioretty got one on the board for Vegas but it wasn't enough.
Stone scored on the power play at 6:36 of the first period for a 1-0 lead, and Reilly Smith made it 2-0 at 8:20.
Boston then scored four unanswered goals.
With Boston's Chris Wagner and Vegas' Nick Holden battling for position in front of Fleury, Krug extended the lead with a blistering slap shot that got through traffic and made it 4-2.
David Pastrnak had one goal and two assists for the Bruins, who started the season on a four-game road trip.
NOTES: Bruins captain Zdeno Chara, 42, appeared in his 1,488th game, breaking a tie with Wayne Gretzky for the 23rd-most in history.
Bruins: At the Colorado Avalanche on Thursday.