That's Philadelphia 76ers head coach Doc Rivers giving a locker-room style speech at a Joe Biden rally on Sunday ... and he's fired up!!!

Rivers -- who was only hired in Philly last month after years in L.A. -- was a featured speaker at Biden's drive-in rally at FDR Park in South Philadelphia.

And, when Doc stepped up to the mic, he didn't hold back!

"We have so much at stake, everybody. We have so much to do still," Doc explained.

"I'm coming up to you as your coach today -- how about that? So, I want you to understand this. We're in the 3rd quarter. The 4th quarter is on Tuesday. We have to close the game on Tuesday."

Rivers -- who used to be friends with Donald Trump back in the day -- claimed POTUS is bad for the country and explained he believes electing Biden/Harris is the best way to repair America.

"This is a country that needs to come together. We have to come together if we want to be great. And the only way we're going to come together is by Joe Biden being our president and Kamala Harris being our Vice President."

Rivers encouraged everyone to vote -- and help others vote!

"And it doesn’t matter how long you have to wait in line — don’t leave that line Tuesday! If you’re there for 1 hour or 2 hours or 3 .… don’t leave that line! You stay the course!"

Rivers also suggested the Trump campaign is using "scare tactics" to prevent Biden supporters from voting.

"You know what bullies do? They start using scare tactics when they have nothing to run on. When they have nothing to say, they try to scare you. Be strong and be mighty and vote."