Aaron Judge is killin' it, but don't go retiring his number just yet ... so says Yankees legend Reggie Jackson, who told us the rookie phenom still has a lot more to prove.

Mr. October was RAVING about Judge when we ran into him at LAX --  saying he's showing serious signs that he could be one of the best to ever do it -- but notes, "One half season doesn't make a great player."

It's been a helluva 1/2 season -- already racking up 32 HRs (tied for the MLB lead).

But Reggie warns it doesn't get easier -- Judge is gonna have to make some key adjustments if he wants to be one of the greats.

BONUS -- Reggie also talks sex at the end of the clip ... by accident.

Still funny, though.