Another part of this is that the other KC player (not Clark) grabbed Russell's right foot at just the wrong moment where it put him out of balance and flying to the ground without use of that right leg/knee/torso to help spread out/cushion the landing impact. Then the landing was a side-impact hit to the head, which was a classic concussion strike, doesn't look so wicked compared to some frontal impact that don't result in concussions, but there is something about the side impact where the brain is more vulnerable.
My first thought was broken collarbone, or shoulder injury, what with Clark landing on him, but it quickly became clear that Russell could move his arms normally. I recall Aaron Rodgers breaking a collarbone, IIRC in 2017, on a play that inspired (IMO) adjustments to roughing-the-passer rules such that a DL landing on a QB with his body weight is now an automatic flag, and it is the pass-rusher's obligation to avoid landing that way during a sack or other hit.
Concussion result is both better and worse at the same time for Russell. Better--he might recover quickly; Worse--persistent concussion could be a career-ender, like what we saw with Steve Young. We really don't know how many mini-concussions Russell suffered in his Seahawks career, but we did see him take some wicked shots over his career.
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