Dataroma
Why has QJ slowed down? Maybe it's partially because of the injury he sustained, but he's also seen a notable shift in route tree these past 2 weeks.
In Week 1-6, he was running horizontal breaking routes at a 32.4% clip, and getting targeted on 27% of those routes. This coincided with a less vertical role, with a 34.6% vertical route rate (0.13 TPRR on those).
In Week 7-8, QJ's horizontal breaker rate dipped down to 22.4%, still getting targeted on 24% of those routes. Even more notable, his vertically-breaking route has spiked up to 64.5% of his total routes (would lead NFL on season-long sample), and he's being targeted on just 2% (!) of these routes.
In layman's terms, QJ has shifted to a field stretching-role after being a key cog in this Chargers down-to-down offense the first month of the season. Unfortunately QJ, while he's able to fulfill this role, he has been much better separating and earning targets on horizontal breakers.
Although the Gadsden emergence is certainly hurting QJ, it's actually been Keenan (46.1%→54.3%) and Ladd (47.4%→57.9%) who have really hurt QJ in the horizontal route aspect.
All data here per the awesome @FantasyPtsData data suite.