That loss last night has a bit of an asterisk. The Leafs got a late powerplay on an "egregious" call (according to Cassidy). They scored, then added an empty-netter. Additionally, Bergeron was drilled into the boards from behind by Shostnikov who only received a minor. Bergeron got up and went after Shostnikov and received a roughing minor for his troubles (if he stayed down it's likely a major and a game instead of coincidental minors). I saw a GIF of the play, it was pretty blatant. Should be getting a call from the dopes at DOPS.
All that said, this is a tired team, just finished a long road trip, and one of their top forwards may not be 100%. Sens have to get the early jump and never stop for 60 minutes.