Benn went to the locker room in the first period and did not return. The 35-year-old was on the top defensive pairing with Morgan Rielly.
Toronto also lost defenceman Carl Dahlstrom with a shoulder injury. The 27-year-old took Benn's spot on the top pairing with Morgan Rielly before leaving the game.
With both defencemen gone, forwards Calle Jarnkrok and Alexander Kerfoot took shifts on the blue line.
Toronto's defence corps was already without Timothy Liljegren to the start of training camp because of hernia surgery and Rasmus Sandin, who is in a contract holdout.
Benn joined the Leafs on a one-year, $750,000 contract back in July.