WATCH: Celtics star Jayson Tatum gives shoutout to childhood friend Matthew Tkachuk after sealing NBA championship

Boston Celtics star Jayson Tatum gave a shoutout to his longtime friend Matthew Tkachuk after the Celtics won the NBA Finals on Monday. Tatum's team secured the championship series 4-1 against the Dallas Mavericks.

Shortly after the win, Tatum had a message for Tkachuk:

"Tkachuk. Get it done tomorrow. One kid from St. Louis winning the championship today. Just make it two tomorrow."

The message was a rallying cry for Tkachuk and the Florida Panthers, who will play Game 5 of the Stanley Cup Final on Tuesday. They currently lead the series 3-1 and have a golden opportunity to win the championship at home.

Tatum and Tkachuk's bond goes back to their high school days at Chaminade College Preparatory School in St. Louis. They have remained close friends ever since then.

A video of the two went viral last year. The video was uploaded on Jayson Tatum's YouTube channel on April 10, 2013. It shows a young Matthew Tkachuk dancing with a laptop in hand. At that time, both were in the ninth grade. Tkachuk pursued hockey in the United States Hockey League, while Tatum completed his education at Chaminade and graduated in 2016 before beginning his basketball career.

Matthew Tkachuk confident ahead of Stanley Cup Final Game 5

Matthew Tkachuk expressed confidence ahead of Game 5 in the Stanley Cup Final. He even accepted his need for improvement after a tough Game 4 loss.

"Looking forward for hopefully my best game of the series tonight," Tkachuk said (via NHL.com). "I thought last game was nowhere near good enough. I'm way better than that. Maybe the last four periods, going back to the third period of Game 3, I can be a lot better."

He pointed out how strong the team is together and their goal to win at home:

"The good part about us is it's not about one guy, it's not about individuals here. We have a chance to capture the biggest goal of our lifetime, so we're [going to] go do that."

Florida coach Paul Maurice praised Tkachuk's performance and accepted the need for consistency from every player.

"He's been good," Maurice said. "...The inside play that he makes to Bennett on the goal is an incredible play, He's very close leading our team in scoring. We just need solid out of each guy. That's what we're looking for."

The Panthers are now focusing on Game 5.