To be fair, he was outstanding on tape last season. Part of the issue has innately been in his usage. It has been categorically versatile, more than you'd prefer at times even. Of his 944 snaps, he's played 466 at FS alone. 107 have come at slot corner, when he was directly lined up against a WR or slot TE, and over a dozen at either LCB or RCB. That's 588, which is 62.3% of his overall usage. You look at a player like Kam Chancellor, and he's statistically played the majority of his snaps at either LB, on the edge or at SS this season. So there has been a tough learning curve for Jefferson. It doesn't completely write off his early season struggles, but it's worth noting.
Jefferson had a fair share of assignment lapses earlier in the year, but it seems he's been asked to play a lot more close-intermediate snaps and he has done very well there recently, leveon bell folded his ass up multiple times but bell is a total freak and Jefferson did enough for the most part to get him down. Aside from his tackling attempts vs Bell he's had a pretty solid 2nd half.
I think he's been very disappointing considering the deal he got, but I think he's gonna be a big part of our future and show why we gave him that deal
i've said since we signed him that my dream alignment involves jefferson at dimebacker next to CJ with weddle on the backend (+ another safety which at the beginning of the year i thought should be webby but at this point is more likely levine or clark) - and using jefferson in those packages has demonstratively helped him to play better you can allow him to go and hunt the football close to the LoS which is really what we signed him for
it's another case of pees getting too cute and trying to out bluff the opposition by using his players in unexpected situations more often (that's all well and good but when you do it all the time it is no longer a surprise and it puts your players in worse spots)