Vikings quarterback Josh Dobbs lost a first-quarter fumble Sunday night on a questionable hit by Denver Broncos safety Kareem Jackson, a play that marked a key turn of events in Minnesota's eventual 21-20 loss.

Watching in real time from the sideline, Vikings coach Kevin O'Connell said he thought that Jackson -- playing in his first game back following a two-game suspension for similar hits -- should have been penalized after lowering his head and driving the top of his helmet into Dobbs' chest and face mask, Kevin Seifert of ESPN reports.

One of the NFL's health and safety spotters noted the contact and sent Dobbs to undergo a concussion check, which he passed. But the Broncos recovered the fumble at the Vikings' 30-yard line and ultimately turned it into a 31-yard field goal from Wil Lutz.

Asked if he thought Jackson should have been flagged, O'Connell said: "On the field, it felt like that. I'll have to go back and look at it. It tends to happen pretty fast out there, but in the moment, coming from the second level the way he did, it seemed like a pretty direct helmet-to-helmet type hit. But I'm sure [officials] saw it differently on the play. They're officiating a fast play right there."

Dobbs was actually a running back on the play; Vikings tight end T.J. Hockenson lined up under center and pitched the ball to Dobbs in an attempt to convert a third-and-1. That means Dobbs did not merit protection afforded to quarterbacks in the pocket.

But NFL players also are prohibited from lowering their heads and making forcible contact with their helmets against opponents, a rule that is not often flagged because it is difficult for officials to see. But such hits often result in warning letters, fines and occasionally suspensions.

Jackson was suspended for three games, reduced to two on appeal, after hitting Green Bay Packers tight end Luke Musgrave when he was in a defenseless position in Week 7.

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