I think people are being a bit rose colored glasses with his rookie year just because relative to the later years it was better, but it was never good.
For being a 6'1 elite athlete with a huge frame and wingspan, he should be a suffocating defender. He should be a guy who can get into a jersey. He has never been that guy, and when you are undersized (under sized in terms of vertical length) you have to play that way and Donovan has never been able to. That and his defensive IQ is really poor. I think his best bet for being a more impactful defender is being in a more aggressive defensive system that encourages him to gamble more so he can at least improve his defensive playmaking ability.