DeRozan's Sacramento Kings were in Toronto to play against the Raptors ... and, despite suiting up for Drake's beloved team from 2009 to 2018, the 6ix God gave him far from a warm welcome.

The night began on a high note with the Raptors' jersey retirement ceremony for Vince Carter ... but when Drake was asked on the TSN broadcast if DeRozan should have his jersey in the rafters, too, he threw some serious shade.

"If you ever put up a DeRozan banner, I’ll go up there and pull it down myself," Drake said.

It didn't end there ... Drake stared DeRozan down every time the six-time All-Star was near him ... and at one point, he appeared to call the hooper a p***y."

During the post-game press conference, DeRozan learned about Drake's banner remarks ... and from the look on his face, he wasn't pleased.

"He gon' have a long way to climb to take it down," DeRozan said. "Tell him good luck."

But the bad blood isn't just a result of the Compton native playing for a different team ... all roads are leading back to his cameo in K. Dot's viral diss track music video aimed at Drizzy -- as well as his onstage appearance at the rapper's L.A. concert in July.

In the song, Kendrick says, "I'm glad DeRoz' came home, y'all didn't deserve him neither."

DeRozan tried to play Switzerland amid the beef ... saying despite his role in Kendrick's visuals, he still had love for Drake.