The guy's got a defensive rating of 112 which is even worse than Rajon Rondo. No thanks.
Two factors in increasing order of importance:
1) He's a rookie -- most rookies struggle to learn NBA defense, especially bigs.
2) Over 68% of his minutes have been with Devin Booker on the floor. Booker is possibly the worst defender in the NBA.
In the minutes Ayton has played WITHOUT Booker, his DefRtg is ~5 points better.