He would have beaten Whittaker if he didn't injure his knee. The fight was there for the taking but he just wasn't active enough and Whittaker landed a takedown. Whittaker is a top level fighter and probably the second best in the division.
He is at the top of the division. He is only 28 and Izzy is 32 (and will probably go to LHW within a couple of years).
He does seem to carry a lot of injuries and his body might completely break down. If that doesn't happen, he has more than a decent chance to win a belt in his career.