The Panthers (33-32-12), who were eliminated from playoff contention when the Blue Jackets defeated the Islanders, have lost four of five and been outscored 20-9 during a three-game losing streak.
"It's just not paying attention to our system, it's getting away from our system," Florida forward Vincent Trocheck said. "Somebody is trying to do someone else's job, you're duplicating on guys, and whenever you duplicate, obviously someone's going to be open. In the NHL, they're going to find that open guy, and they did, especially in the first, and that's what leads to goals-against.
"And it seems like we get down two goals, just like the last couple games, and we let off the gas, kind of just quit. It's embarrassing, it's extremely embarrassing. I mean, every single guy in here should be embarrassed the last three games."
James Reimer allowed three goals on nine shots before he was pulled in the first period of his first game since Feb. 26. Sam Montembeault made 15 saves in relief.
Tatar gave the Canadiens a 1-0 lead at 6:58, and Brett Kulak made it 2-0 at 12:39 with a one-timer from the slot.
Montembeault replaced Reimer at 14:28 after Tatar scored his second goal of the game to make it 3-0.
Artturi Lehkonen made it 4-0 with his 10th goal at 2:52 of the second. Tatar got his NHL career-high 57th point with an assist on Phillip Danault's goal at 12:40 to make it 5-0.