You're right that those numbers are from PFF. I have no idea how they count the snaps but here's detailled information on their take of Whitehead's alignment vs. the Rams:
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Those breakdowns are more than a little strange and
very interesing. It looks like they go by where the player is positioned relative to the offense, relative to the LOS, what type of player he is lined up over, and who the guys are he's playing next to. If I have to guess, I don't think they factor in what the guy does post-snap.
13 snaps as a D-Line OLB, for example? That's a 3-4 edge LB/rusher. I think you'd be hard pressed to find those snaps in that tape the way we understand 3-4 OLB. But he just might be standing in approximately that spot, on the line at 9-tech or wide 9, not over a receiver, whereas he is actually there to edge run defend or looking for a RB release. Or maybe he just drops in the short zone. I don't think you're going to find him rushing the passer from that spot very often in this tape, if at all.
But this breakdown goes to my point. The ILB / S distinction is largely artificial with these hybrid players just as the 3-4 / 4-3 distinction is artificial on a wide range of snaps. Looking at the 33 snaps designated as a box LB, only 8 of them are counted as a traditional 3-4 ILB designation (RILB and LILB) while the other 25 are traditional 4-3 designations (MLB. RLB, LLB)!
If we must apply the artificial S / LB designation, we have to consider that on a preponderance of these snaps Martinez is the only guy on the field we would traditionally call an off-the-ball LB if we don't give Whitehead that designation on a wide range of these snaps.
If we have to take these 75 snaps, and slot them into either a DB or LB category while realizing a hybrid player is neither, the only ones that clearly fall into the DB group are the 18 (24%) at wide corner and FS. From my examination of less than 1/3 of the snaps it would not surprise me if those counts are sensible, set 15 yards deep at S or wide on a WR or TE. That 24% is probably as close as you can get to "DB" in nickel or dime as we traditionally think of those sets.
We tend to think of the hybrid ILB as a smaller, faster LB better able to cover TEs and RBs with sideline-to-sideline speed for run defense. That's a big part of it, but not all of it, especially in this "everything including the kitchen sink" defense in this game and in general this season. It's probably time for a "hybrid player" category, though for the life of me I'm not sure how those players would be clearly identified. Anyway, they mark Whitehead as a "safety". For this game anyway "ILB" would be better if we must choose.
Anyway, they sure put a lot on Whitehead's plate for a guy with so little experience. It would appear they've found him to be a pretty smart ballplayer. I think he did a pretty good job except for that Gurley 30 yard TD reception. I don't know how PFF graded him, but they do seem to heavily weight big plays, good or bad. Whether this was a game plan/matchup decision in lieu of Burks or the way they'll go forward we'll just have to wait and see.
According to the NFLPA's website the Rams are currently $450K under the cap.
In the 7 hours between my post and yours the NFLPA updated their web site. We know a team cannot go over the cap in-season controlled by the league office contract approval process. The most likely reason they would show a negative cap space number at midnight and a positive number at 7:00 AM is a lag in processing an offsetting transaction that added to the cap space. Who knows...a guy or gall has two files dropped in the in-box, processes the one transaction at 4:00 PM, goes home, then comes back at 7:00 AM to finish up with the offsetting transaction. There are a variety of possible explanations.
Whether the Rams have zero cap space or $500,000 in cap space does not materially affect my cap analysis, one of these distinctions without a difference.