Conor McGregor Cried After 1st MMA Loss, Opponent Explains Why ...

Conor McGregor wasn't always a monster in the cage -- he tapped out in his 3rd-ever MMA fight and wept in the cage ... but his opponent says the dude is FAR from a cry baby.

We spoke with Artemij Sitenkov -- the very first man to make Conor tap back in 2008 after putting the Irishman in a leg lock during a Cage of Truth fighting event in Dublin.

"He was 19 years old ... and it was his first defeat, so yeah, he was emotional."

You can see in footage from the event, Conor was shocked after the match -- sitting on the cage floor in disbelief.

But, Artemij says the tears don't make him any less of a man -- explaining, "When I was a kid and I lost my first fight, I was also crying. It's natural."

Conor's a different fighter now, according to Artemij -- "I didn't beat superstar McGregor. I beat teenager McGregor."

Artemij is still very involved in MMA -- he currently coaches young fighters ... and gave us a prediction on the upcoming Conor vs. Khabib fight.

Spoiler alert ... might wanna get the tissues ready.