Terrifying moment: NFL star Marvin Harrison Jr struggles to his feet after a brutal blow to the head… before being quickly ruled out with a concussion.

Cardinals rookie Marvin Harrison Jr was left dazed and briefly unable to stand upright after suffering a concussion from a brutal blow to the head.

The wide receiver, the fourth overall pick in the NFL draft, had to be helped off the field after his helmet collided with Green Bay linebacker Isaiah McDuffie's thigh.

In worrying scenes, Harrison Jr could be seen standing up before stumbling and falling to the grass as McDuffie looked on worriedly.

The 22-year-old, son of Hall of Fame catcher Marvin Harrison, was removed for evaluation in the injury tent.

Harrison Jr finally entered the tunnel before the Cardinals confirmed his afternoon was over.

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Harrison Jr stood up but tripped and fell to the grass before being ruled out.

“WR Marvin Harrison Jr. (concussion) has been ruled out,” Arizona said on social media early in the second half.

Harrison Jr is one of the most exciting prospects in football. He had 279 yards and four touchdowns on 17 receptions in his first five NFL games.

In Sunday's 34-13 loss to Inexperienced Bay, Harrison was targeted twice by quarterback Kyler Murray but failed to make the catch.

The Cardinals also ruled out defensive pick Bilal Nichols with a stinger. Nichols had one tackle in the game and 10 tackles in six games this season.