Joshua lost via TKO at the hands of Andy Ruiz Jr. at MSG Saturday night ... in one of the most shocking boxing upsets of all-time.

The loss made a lot of people question Joshua's greatness ... and whether he's truly elite.

Not former 2x heavyweight champ George Foreman ... who actually believes the loss could HELP the UK superstar become an even better fighter.

"They're gonna have to go back to the gym. I think he'll be a greater fighter, Joshua now. If he goes back and gets his mind together. He's gonna even be better, but right now he better learn to box and he better get a good manager. I'll do it."

We asked Foreman if he was serious about the offer ... and his response?

"That's certain. He needs someone like me."

If Foreman was steering AJ's career, there's 1 bigtime move he'd make straight away -- NO REMATCH.

"If I'm Anthony Joshua's manager, I say, 'let sleeping dogs lie.' I don't want to fight that guy anymore, because you don't know what you did wrong. You go back and you're gonna do the same thing. Let it slide" ... Foreman tells us.

"Leave Ruiz alone. Let him alone."

So, does the loss take Joshua out of the conversation with Deontay Wilder and Tyson Fury for best heavyweight in the world? Foreman's answer ... 6 emphatic "no's".

"No. No, no, no, no, no."

"One punch, uh huh. One punch is not gonna change that, or one knockdown is not gonna change that. He'll be back. He's in the mix. And, probably he'll be better."