The Minnesota Vikings weren't expected to put up much of a fight against the San Francisco 49ers on Monday Night Football. The seven-point underdogs just had other ideas.

The Vikings jumped out to a 10-point lead in the first half and held an advantage late into the third quarter. So maybe a touch of frustration on the part of the 49ers explains the apparently dirty play caught by ESPN's cameras

Fans on Twitter picked up on 49ers linebacker Dre Greenlaw appearing to knee Viking center Garrett Bradbury after a play.

Dirty Play: 49ers LB Dre Greenlaw appeared to knee Vikings C Garrett Bradbury

If Greenlaw wasn't trying to drop his knee into Bradbury on purpose, it's hard to imagine what he was trying to do there...It sure looked like a dirty play.

Fans on Twitter agreed, with some flabbergasted he wasn't called for a penalty with the ref looking straight at him while others wondered if he could face punishment from the NFL.

We've seen the NFL take action against players for dirty plays. And this one could definitely end up getting a look from the league office.

The NFL can't have players perform that kind of extracurricular. In this case, it doesn't look like the center was hurt but it could have led to an injury. There's no room for that in the game. Be physical up until the whistle, but don't be underhanded and don't take cheap shots that could injure an opponent.

Greenlaw also put his team in jeopardy with that stunt. He is lucky the referee didn't penalize him and the Niners for 15 yards.

Greenlaw had seven tackles and one QB hit in the first three quarters against the Vikings. He was making his return to action after missing the game against the Browns with a hamstring injury.