Khan bowed out in the 6th round of his fight with Terence Crawford this weekend after getting hit below the belt ... insisting he was seriously hurt, and couldn't continue.

But, fans felt cheated by the ending ... and they were pissed.

And, they damn sure should be, according to Roy ... who says Khan flat out waved the white flag.

"Amir Khan quit."

Jones Jr. -- a multiple time world champ across 4 weight classes -- says he would NEVER quit inside the ring.

"When I go in the ring. Carry me out. I don't wanna go out on my feet if I lost. Carry me out. I rather give it all, or get it all taken away from me."

Still, RJJ says all ain't lost for Khan -- a fighter he likes and respects -- as long as he gets real about what happened.

"Amir, I love you as a fighter. I respect you since you were an amateur fighter. You went up 2 weight classes, took on Canelo. You did a lot of beautiful things to show that you have courage," Jones Jr. says.

"This time you were in a no-win situation, and you refused to continue," Jones tells us ... "You stopped at a time we know you could've given more. You didn't give it to us, the fans, your all."