Nas was also vicious and had a deadly punch. Not to mention his chin. I don´t think Whittaker could survive a war like Nas vs Kelley.
By the time Naz fought Barrera, Naz's hands had so many injuries and ops, they were practically like chalk, plus he crashed a huge amount of weight in the week prior to the fight due to a whole host of reasons including more niggling injuries while Barrera was absolutely at the peak of his powers.