Let's take a look at San Francisco's defense.  The table below lists the player, their PFF grade, their snap percentages for the season, and their playing percentages in week 19 and 18.  The snap counts come from nflgsis game book statistics, and the season-long snap information came from Pro Football Reference.  The table then provides the traditional statistics of combined tackles, pressures, and quarterback hits, all of which were derived from PFR.  Note that PFR is not as generous with crediting players with pressures as PFF seems to be.  To give context, I provided Rashan Gary's and Kenny Clark's PFR statistics to compare to the edge players and interior defensive linemen.  I have not included sacks because I think pressures are more important than sacks.  The analysis is based solely on available statistics: no film study was involved. 

Nick Bosa is an elite edge player.  Samson Ebukam starts on the other side and is good.  San Francisco has a host of good edge players so they can keep the starters fresh.  Arden Key has a lot of quarterback hits for only playing a third of the time.  Charles Omenihu was a one-man wrecking ball against the Cowboys with 3 quarterback hits and 1.5 sacks.  The group as a whole can get after the quarterback.  Bosa finished with 15.5 sacks, Key with 6.5, Ebukam with 4.5.  Omenihu actually did not have any sacks during the regular season.  They all had good numbers for missed tackles.

Armstead and Jones are the defensive tackles.  Both are very good, with Armstead (6 sacks) being particularly adept with his pass rushing.  The backups are not nearly as good.  Although Maurice Hurst has a fine PFF grade, the sample size is small, and he was Inactive in week 18 and 19, two must-win games.  Javan Kinlaw probably was supposed to be in the rotation but he has been both bad and injured.  In any event, Kinlaw is on IR and will not play.    

Shazeez Al-Shaair and his backup, Demetrius Flanigan-Fowles are the Sam linebackers.  Fred Warner is the Mike linebacker and has no obvious backup.  He played every down, essentially.  Dre Greenlaw and his backup, Marcell Harris, are the Will linebackers.  Greenlaw played in weeks 1 and 2, and then returned full-time for weeks 18 and 19.  The starters are good but the depth is non-existent.  2.41 linebackers play on average per snap. 

San Francisco desperately needed to win in weeks 18 and 19, so the snap counts for those weeks indicate who the team wanted on the field.  Despite the nice PFF grade, when San Francisco had to win, Flanigan-Fowles played just special teams in the playoff game and just 9 snaps in the final regular season game even though the starter, Al-Shaair, was inactive due to injury.  Flanigan-Fowles, a former UDFA, has been in the league since 2019.  Marcell Harris injured in achilles in practice prior to their week 18 game against the Rams and has not returned to the field.  Harris sports a 17% missed tackle percentage, which probably explains his terrible PFF grade.  He has a history of missed tackles.  He does have good coverage statistics this year, but they were terrible in 2018 through 2020.  His PFF grades have been in the forties in three of his four seasons.  It remains to be seen whether he will play.  

Fred Warner had an elite year in 2020 (All-Pro honors), but he came back to earth in 2021, playing more like he did in 2018 and 2019.  He is a quality player with sideline-to-sideline range.  He did miss some snaps against the Cowboys late in the game due to an ankle injury.  His passer rating allowed crept up.  He was targeted 62 times.  Dre Greenlaw was a 5th round godsend for San Francisco as he was able to start as a rookie back in 2019.  He is good in coverage and is a sure tackler.  In the absence of Greenlaw, Al-Shaazir, despite being unaccomstomed to the role, had to spend a good deal of time in coverage in 2021.  It could have been worse.  Al-Shaazir, a former UDFA from Florida Atlantic, is a little undersized for a Sam at 6'2" and 228 pounds.  His missed tackle percentage is okay.  Robinson and Nzeocha are practice squad members who were promoted since bodies were needed.

Emanuel Moseley missed six games but he plays every snap when he is healthy.  He is a sound player.  Ambry Thomas had been sitting behind Josh Norman, but Norman was so terrible that San Francisco put Thomas, a rookie third-round pick, on the field to sink or swim beginning in week 14.  He sunk for a while, but has been given better than average grades in his last three games by PFF.  San Francisco fans have described his growth as meteoric.  Thomas is still a rookie who has been up and down, so expect the Packers to test him.   Deommodore Lenoir is a fifth round rookie who has gotten some snaps this year, but he has fallen out of favor due to missed tackles and ineffectiveness.  I am not sure what happened to Lenoir in week 18: perhaps he was on the covid list that week.  He is still on the roster at any rate.  Dontae Johnson is an old pro who has bounced around the league, never being the guy.  He also has missed too many tackles.  K'Waun Williams is listed as a nickelback on the forty-niners website.  He missed essentially 4 games.  When he is healthy, he plays far more than the 60% of snaps listed in the table.  He is an old pro, slightly built, who has had issues with missed tackles over the years, though this year he has been adequate in that department.  Dre Kirkpatrick was released in week 10.  Darqueeze Dennard was called up from the practice squad and played 21 snaps in week 18.  His sample size is tiny.

jimmy Ward is the starting free safety.  He is a quality player.  He does not miss tackles and his coverage is pretty good.  Jarrod Wilson is listed as Ward's backup but has played just 8 snaps.  Jaquiski Tartt is the strong safety.  He has been an average to just above average starter for several years now.  His missed tackles are up a little this year, but that has not been a problem for him over the years.  He often plays every snap but has had games in the 60% range.  Tartt was limited all week for the week 18 game and was listed as questionable.  Talanoa Hufanga backs up Tartt.  He is not bad.  He started 3 games earlier when Tartt was out and filled in when he was returning from injuries.  

For context, the table lists Douglas and Amos.  I see nothing in the traditional statistics that explains the relatively low grade for Douglas.  His pass rating allowed is excellent and his missed tackle percentage (8.1%) is fine.  Amos' passer rating allowed crept up from 85.5 and 85.7 in 2019 and 2020 to 101.4 this year.  His missed tackles percentage (7.9%) is very respectable.  

Omenihu (41.8), Dontae Johnson (44.3) and Jimmie Ward (51.0) were the three lowest graded players against the Rams in week 18.  Ward bounced back against the Cowboys with a 72.9 grade, and cornerbacks Ambry Thomas and Moseley had grades of 73.4 and 69.7.  Jaquisti Tartt and Kentavius Street had rough day against the Cowboys according to PFF with grades of 48.6 and 50.9.

There you have it.  It is up to Matt LaFleur and Nathaniel Hackett to devise ways to exploit the San Francisco defense.