It all happened on Richard Sherman's interception of Kirk Cousins, which resulted in a return opportunity for Sherman and turned defensive end Nick Bosa from pass rusher to lead blocker. He found O'Neill, who did not see Bosa before the defensive end hit him with a blindside block, drawing a flag and leaving O'Neill on the ground in obvious pain.
NFL Network's Tom Pelissero described a scene in which Vikings players were "livid" about Bosa's hit on O'Neill, with multiple players yelling at the 49ers. Vikings coach Mike Zimmer agreed after the game that the hit was a "cheap shot."
Niners coach Kyle Shanahan disagrees.
"I think people say 'cheap shot' when somebody gets hit violently and to me 'cheap shot' means your intent is to try to hurt someone," Shanahan said Monday, via NBC Sports Bay Area. "And I don't think he's thinking about that at all. That's a normal block in football for a long time. The guy wasn't completely out of the play. It was a guy who could've made the tackle, and Bosa went and hit him. That's a rule. You can't do that right now. You can't hit a guy in that position."
The flag was appropriately thrown, O'Neill left under his own power and the right tackle did not return. It's worth pointing out Shanahan did agree the hit was a penalty, he just didn't think it was a cheap shot or dirty play. That's left to interpretation in a world full of opinions.
Defenders are taught to put a body on a body in the instance of a sudden-change situation like an interception or fumble recovery. Defenders become lead blockers with the goal of turning the takeaway into a touchdown. Bosa did that with his block. He just did so illegally.
"Basically got to set a screen, which means let him hit you and just take it," Shanahan said when asked how to block legally in such a scenario. "That's part of this game. That's what you got to do. And that's what we'll coach him up to do because you will get a penalty for that. But to say it's a cheap shot means you're trying to hurt someone and stuff, and I know that wasn't the case."