Borowiecki was assessed a major penalty for boarding on the play early in the first period of Saturday's game.
Toffoli left the game briefly before returning to the bench. The Kings scored twice on the 5-minute power play.
Upon his return to the ice, Borowiecki was challenged to a fight with Kings defenceman Matt Greene.
The Senators continue their west coast road trip against the Anaheim Ducks on Sunday.
Viewewrs in the Senators region can watch the game live on TSN5 and TSN GO at 4pm et/5pm at.