It's not that Nnandi doesn't cover number 1 WRs, it's just that he always plays RCB. If the number 1 lines up on the left, he covers him.
Nnandi and Revis are both special players.
Using 2009 as an example, Revis covered all number 1 WRs on the opposing team and when he was thrown at, QBs had a rating of about 32.0. That was Brady to Moss, Schaub to Johnson, Manning to Wayne (for one half), Ryan to White, Matt Moore to Steve Smith, Brees to whomever Revis covered that day, etc. Revis was thrown at as much as any CB in the league and held those combos to a rating of 32.0. Was he helped by a good pass rush? Yes, but those numbers are ludicrous. He made those guys worse than Jamarcus Russell.
Now also in 2009, Nnandi was thrown at less than 2 times per game (Revis saw over 120 passes his way) . Nnadi was right around 30 throws his way. That's legitimately shutting down half the field. Obviously other teams were showing him serious respect because he's that good.
I just think Revis is a once in a generation talent to do what he is doing in the most pass happy time in league history.
And for the record, Routt had an outstanding season last year. He should have a high rating.