They are 15th IRL. If you are talking about the game's ratings then yeah, clearly that is amiss.
Based on the ratings I posted, Dirk is absolutely the better defender than Kleber in game. Dirk should match on Kanter IRL.
In general a low-usage offensive player is going to be a defense/hustle guy. He's the 2nd best shot blocker on the team among the healthy bigs (3.6%, behind Mejiri), and ranks 18th among PFs in DRPM (+1.54).
2K's defensive ratings are absolute crap (which ties into why the CPU alternates between OP or trash at defense). These ratings aren't plug & play like counting stats and 2K doesn't evaluate them anywhere near as frequently as they should.