Watch: A.J. Greer's hit on David Pastrnak prompts Pavel Zacha to drop gloves against Panthers forward

It's an unwritten rule in professional hockey that star players are off-limits for a targeted hit, and anyone who violates that must pay the consequences. During Monday's game at TD Garden between the division rivals Boston Bruins and Florida Panthers, NHL fans were treated to a prime example.

Panthers forward and ex-Bruin A.J. Greer leveled Bruins sniper David Pastrnak from behind along the boards, immediately prompting a response from Boston's Pavel Zacha. He dropped the gloves and tussled with Greer, who would eventually get the better of the exchange.

Still, the message was sent.

For Zacha, it was the second fight of his NHL career. Ironically, his previous fight came against a future teammate Nikita Zadorov, then a member of the Colorado Avalanche; Zacha played for the New Jersey Devils at the time.

Meanwhile, Bruins captain Brad Marchand also stepped in to defend Pastrnak, tussling with defenseman Nate Schmidt and taking him down to the ice after he felt the Czech forward was being targeted.

“Pav did a great job getting in there for Pasta,” said Marchand, according to Boston Hockey Now. “I felt that guy was going after Pasta too. Those things happen. Its something guys have done for each other. I think Pave did a great job there.”

Unfortunately for the Bruins, the Panthers beat them for the second time in the last week. Florida earned the two points with a 4-3 win, giving them an undefeated record against Boston this season, including a 6-4 victory at Amerant Bank Arena last Tuesday to begin the season.

Bruins coach Jim Montgomery loved the physical response against Panthers

Bruins head coach Jim Montgomery expressed his appreciation for his team sticking up for one another during the matchup against the Panthers, who eliminated them from the postseason the last two years.

“Their winning the mental side of the battle,” said Montgomery postgame. “Pasta got hit. And I love what Pav did. He went in and defended him."

However, Montgomery was also quick to call out Boston's undisciplined play which resulted in multiple trips to the penalty box.

"That’s great, but the penalties after that, that’s not disciplined. In all four games we’ve taken at least four minors," he continued in his postgame press conference. "We can’t win hockey games that way. And we can’t lose as many battles as we do. They are the standard right now in the NHL for winning battles. We’re not up to that level yet. We have to vastly improve.”

The Bruins continue their schedule by traveling to the Rocky Mountains to face the Colorado Avalanche on Wednesday evening at Ball Arena in Denver.