Single jabs are almost impossible to counter IRL, unless you're running in behind it like a madman. How I've been dealing with (the very rare) jabbers I've encountered is to try and confuse the distance with feints, stand right on the very edge of kicking range and feint then throw, the jab falls just short and you land a crushing low/body kick. I don't have too much experience against it though, because the AI doesn't really do it and 99% of people online are Diaz 1-2 spammers or Tony Ferguson rolling for leg locks.