I'd wager Diaz has better possession stats, which the poster you quoted uses as the bible.
Nevermind Diaz has a 10-cent head and isn't used in any sort of situation that could be mistook for dangerous.
We aren't talking a small difference, they are leagues apart. Add on Klein's expensive contract (for a 3rd pairing D), and Diaz is a more efficient option.
Klein can't be trusted against even average competition as he gets completely overwhelmed. If he gets completely sheltered like he did in the playoffs he is fine, but you don't want to pay someone as limited as that $2.9M/year.