Just some background on the trade transactions/rumors that might put into more pespective, or at least my amateur take on them. There are three coaches are pretty intertwined due to their work together, close friendships, and (somewhat) shared offensive philosophy.
- Sean McVay (Rams)
- Kyle Shanahan (49ers)
- Matt LaFleur (Packers)
McVay and Shanahan were assistant coaches in Washington; LaFleur was Shanahan's assistant in Atlanta.
The offense that Shanahan and McVay use involves a lot of outside runs, or sweeps, and is rooted in Mike Shanahan's old zone blocking run scheme with Denver. The philosophy around it is to force the defense into a simple concept involving isolating a linebacker against the running back; this matchup is critical because it often forces good defenses into its worst vulnerability. Hence, like Mike Shanahan's old Denver offenses, you see a lot of home run plays from RBs in San Francisco, much moreseo than with other teams, as it isn't uncommon to see them break off 80-yard TD runs.
We saw the concept used in Atlanta in 2016 under Shanahan if you think about those big outside runs by Devonta Freeman. He brought it to San Francisco and brought over Tevin Coleman while also plugging in guys like Matt Breida and Raheem Moestert. The ideal running back is a very fast/quick one that excels at a single cut and then explosive vertical acceleration and speed. This offense can cause fits for normally good defenses; we saw the Patriots struggle with Mike Shanahan's scheme for many years despite the Broncos having sub-par quarterbacks. In Los Angeles, Todd Gurley was an ideal (but very expensive) running back who almost won MVP in 2017.
Because the offense uses a pretty unique concept and often the same formation, the matchups it causes with wide receivers and tight ends is usually somewhat predictable. Defenses can change matchups and the routes are often different, but it isn't as complicated as most offenses in terms of a huge amount of combinations and variations. So before the play, the defense isn't in total suspense at what's coming. Stopping it is a lot harder, though. It is kind of like an RPO in that sense, where it boils things down to simplicity, but it's a little different because there's less decision making on behalf of the QB.
And that's why the QB responsibilities for the Rams, 49ers, and Packers is different from most teams. Kyle Shanahan's big contribution to his father's concept is integrating it with the modern passing game. He uses a lot of predictive analytical models, so before the play, he's broken down the chances of which decision the coach/defender will make and the odds of success for each option. Thus, quarterbacks in these systems generally don't have a lot of improvisational responsibilities and reading a defense on multiple levels isn't a highly valued skill. It was clear when Jimmy Garoppolo merged with Kyle Shanahan in 2017 that, while they won a lot of games, it still wasn't really the right system fit for Garoppolo, who excelled in his short time in New England in a read/react system.
So who are the ideal quarterbacks? Guys who are really accurate, have good arm strength, and appear to be more robotic in their approach. Run the play, throw the ball to player X or possibly to player Y because they are the open route by design. Matt Ryan was a perfect fit for the system, as he is someone who struggles to buy time in the pocket and improvise but is money when he only has to throw to the right spot. Matt Stafford would be absolutely ideal as well with his arm strength and accuracy, not having to look off defenders but just knowing where to put the ball. Kirk Cousins is another guy who would be an ideal fit, for the same reasons, that he's very coachable, does the right things, makes the right throws, even has good stats, but lacks the improvisational skills.
Notable: Ryan, Stafford, and Cousins all have good statistics, don't really struggle with a specific aspect of the game, but find themselves on losing teams and are frequently criticized as either being the problem, or at the very least, not part of the solution. And a lot of that has to do with their very structured, "safe" approach to the game that to fans often looks like they don't care about winning when in fact they just take a "by the book" approach to the game. They'd fit in well with the Shanahan offense.
This is also likely why the Packers likely drafted Jordan Love in the first round, knowing that while Aaron Rodgers is an all-time great player, paying $40M to him doesn't necessarily make sense when there are other needs and LaFleur might get 80% of the same success from an accurate, though less spectacular quarterback.