Ex-K.C. Chiefs superstar Larry Johnson says Kareem Hunt could end up being the best running back in franchise history ... telling TMZ Sports the sky's the limit for the budding rookie.

We spoke with the 2-time Pro Bowler about Hunt's breakout 246-yard, 3 TD performance against the Patriots on Thursday ... and L.J. says the kid is destined for legendary status ... as long as the team stays together.

"I just hope that the business side of Kansas City doesn't screw it up for him."

Johnson says it "brings a tears to (his) eyes" to see Hunt performing so well in his old #27 jersey ... and says he's a serious contender for Rookie of the Year.