While that's true Whitehead didn't perform at a high level and wasn't good enough to be retained.
To change the subject, I have a hard time believing that was the issue.
In the Rams game, Whitehead took 75 of 78 defensive snaps, playing all over the field, ILB, S, slot, perimenter corner. They must have liked him to that point. Jones got zero defensive snaps in that game, perfectly healthy, taking 24 special teams snaps. Josh Jackson took only 2 defensive snaps in that game, perfectly healthy, taking 18 special teams snaps.
Clearly, they thought quite a bit of Whitehead if they were going to let him play perimeter corner on some snaps with Jackson on the bench, and they certainly thought more of him than Josh Jones who sat on the bench all day while Whitehead was taking safety snaps, not to mention the ILB snaps that Jones was supposed to be taking when he was drafted.
You mentioned PFF gave him a bad coverage grade because the opponent had a perfect QB rating on 5 throws, I think it was, into his coverage. Well, 5 throws is a pretty small sample size to start with, and certainly a small sample size on which to base cutting a guy that you gave these snaps to in this difficult assignment, playing him over your two second round picks.
Goff threw 35 balls that day, was sacked 5 times, and he had a couple of runs that were not kneel downs if memory serves. If Whitehead did draw 5 throws, that's less than 1/8 of the drop backs. It's not like they were picking on him all day. And it's not like he had any prior money game corner experience to build on.
Flash forward one week to the New England game. Whitehead took 15 of the first 16 defensive snaps. Clearly his Rams game performance was not a factor in his being released regardless of what PFF had to say about it. To the contrary, they must have thought his performance was pretty decent under the circumstances to give him another go-round of starter snaps.
Then he slapped a guy, got ejected, and was cut the following week.
Was he cut because he "wasn't good enough"? No. He was cut because he wasn't good enough to not be made an example of. And there's a good chance there was some prior (or subsequent?) non-public incident(s) where this was a third strike.