"I thought he was trash," Hill said of his first impression of Mahomes at their first training camp together in KC. "I ain't gonna cap. I ain't even gonna cap."

Hill appeared to remember the moment vividly when talking about it all with the guys on Showtime's "Inside the NFL" this week ... saying he wondered out loud what his team's personnel department was doing after seeing Mahomes live for the first time.

"I was like, 'This who y'all drafted right here?'" Hill said with a smile. "'This who y'all drafted?!'"

Of course, Mahomes ended up turning things around -- he's now widely considered the best quarterback of this generation -- and Hill says it's all 'cause the dude put in hard work.

"He proved me into a whole, complete -- I don't know," Hill said. "Like, it's just that second year, like, his quarterback mechanics were different. It was like he was spending more time with his quarterback coach. Spending more time with coach [Andy] Reid. Like learning the offense."

"It was like, dude is like different!"

Mahomes appears to be on his way to winning his second league MVP award in just his third season as a starter ... he's thrown for 3,497 yards already this year with 30 passing TDs and just 2 interceptions.

And, Hill's on his way to a career year because of it all as well -- already scoring 14 total TDs in just 11 games!!