It's simple: he lost speed.
Look, Rhodes wasn't a Shut Down CB because he interfered on every play. That narrative is patently ridiculous. As bad as Refs are they're not going to allow a DB to hold all the time. Especially not one that is singled up on Julio Jones or Megatron or AB or Alshon. And you know opposing Coaches would have been in the ear of the Head Ref before and during the game screaming for calls.
Reality is a bit more complicated. Xavier was Fast with a capital F. In 2017 he was clocked with the fastest MPH of any player in the League that season during a Pick 6. Any player in the League that year. That's special stuff.
Rhodes always had the speed to stay with even the shifriest Slot Guys. But he never had great technique. Goodish, not great. So he used his physicality combined with speed to disrupt guys at the top of their routes. And he got away with it because A. He was right there with the Receiver B. He was subtle about it and C. The rules weren't being rigidly enforced.
But with the loss of that speed he's A. Not near the Reciver B. Very obvious with his grabs since he's not close and C. The Refs are calling more holds anyway.
So the rules changing exacerbated his over reliance on playing physical instead of with technique. That makes sense. But where did the speed go? Pumpf and others have noted how much thicker Xavier has gotten the last 2 years. He's also been injured. A lot. Real injuries. Because he tackles like Sendejo, throwing himself at ball carriers instead of using technique to wrap them up.
Got too big, takes way too much punishment because of his lousy tackling technique and POOF, speed is gone. And so is the confidence. And likely his career. A team would have to outright ignore the 82% completion percentage against him to trade anything for him. Maybe Chucky would, but the rest of the League is way too into analytics to do that.
I imagine part of Zimmer's frustration stems from the fact that he's always had Corners with good Technique. He gets to Minnesota and finds maybe the most physically gifted guy he's ever had but the kid won't refine his technique. Or maybe let it backslide after Newman retired.
It's not unheard of for a great CB to fall off a cliff. Where's Josh Norman? On the bench is where.
But there's another legit tragedy happening in the Secondary that doesn't get enough attention. Trae Waynes is in the bottom-third of the League as well. Not as bad as Xavier, but not good. With teams actively targeting Rhodes and yet another year of experience under his belt Waynes should be having a career year. So what the hell is going on with him?
Now Mike has not 1 but 2 CBs playing with crap technique which utterly compromises his Defensive Schemes. Coach Z has ALWAYS had at least 1 if not 2 CBs going all the way back to Dallas that he could trust. Now he's having to trust kids while also implementing a lot of zone looks to counter misdirection and pick plays. Kids in age, technique AND learning a hybrid system. Ugh.
Tough spot for Mike to be in. Can't trust his veteran CBs for maybe the first time in his career. Kids aren't ready yet. If he blitzes too much and doesn't get home he knows somebody's going to get beat deep. If he doesn't blitz and the Front Four doesn't get pressure somebody's going to get beat deep.
Imagine if Hughes hadn't got hurt last year where he'd be at developmentally. Or if Waynes hadn't regressed. Or if Rick & Mike had hit on a 3T in either John's or Holmes. If even 1 of those 3 had happened this would be a typical Zimmer Top 10 or even Top 5 Defense and Minnesota would be a legit threat to win it all.