Terence Crawford tells TMZ Sports ... Amir Khan was SMART to quit in the 6th round -- because the beating was only gonna get WORSE for the fighter.
We spoke with "Bud" about that smackdown he put on Khan on Saturday -- in a fight where Khan quit after taking an accidental low blow.
Crawford says Amir essentially "took the easy way out" -- because Terence was finally starting to open up and the beating was only going to get worse.
"I honestly feel like [Amir's trainer] stopped him before anything bad happened to him.