Dolphins rookie linebacker Raekwon McMillan reportedly tore his ACL.
While covering a punt in the team's preseason opener against the Atlanta Falcons, McMillan collided with teammate Deon Lacey.
McMillan was taken by the Dolphins in the second round out of Ohio State where he had 275 tackles and six sacks over three years.
Last season Miami was 29th in the NFL in yards allowed and 30th in rushing defense. McMillan was projected to start for the Dolphins this season.