I was wondering where you were going with this in the title, but it was Nick Diaz's brother Nate that won the Lightweight TUF 5 season.
That would change my answer completely, because if Mac and Nate fought, I would give the victory to Mac, simply because he has been very impressive in his career, and he just seems like he would be too much to handle for Nate at this point in his career.
On the other hand, if Mac were to fight Nick, I feel that Nick would come out with the win. Nick beat Gomi, Lawler, etc., and although Mac has experience, his only true test on a big stage to my knowledge is a loss to Hayato Sakurai at PRIDE 33 earlier this year. If Nick doesn't get cut again, I see him getting the victory.