The 17th overall pick in the NFL draft tore the anterior cruciate ligament in his left knee while playing on the kickoff coverage unit during the Vikings' preseason opener Aug. 14 against Oakland.
Greenway was placed on season-ending injured reserve the following day and could undergo surgery as early as this week.
The injury is a blow to the Vikings' depth at linebacker and also takes out a player who would have been expected to contribute significantly on special teams.
Greenway did not have a starting role at the time of his injury and was being used exclusively as the backup to E.J. Henderson at weak-side linebacker. Henderson has had a standout training camp, so it was unlikely Greenway was going to be in a spot to win a starting job unless he changed his position on the linebacker corps.
This isn't the first setback Greenway has experienced early in his tenure with a new team. During the spring of his redshirt freshman season at Iowa, he tore the ACL in his other knee. However, he was able to return during the fifth game of that season.
"You come back and you're actually faster and stronger, and that's through the rehab and the hard work you use through the four months or six months to bring yourself back," Greenway said. "That happens in a lot of cases with the ACL."
Greenway was injured on only his third special teams play against the Raiders. He also had been on the field for one defensive play as part of a four-linebacker package used in short-yardage situations.
Early in the first quarter, he made a nice shoe-string tackle of Oakland punt returner Chris Carr. That limited Carr to a 7-yard return. But with 4 minutes, 11 seconds left in the quarter, Greenway awkwardly twisted the knee while bracing under a block.
"It was just the way my body was positioned," Greenway said. "There are a lot of physical things going on. People get hurt all the time. It stinks."
But Greenway quickly switched to a more upbeat tone saying, "I'm excited about the fact that I have a chance to come back. In a situation like this, you need to find as many positives as possible."
CAMP CALENDAR: The Vikings broke camp on Friday and will now practice at their Winter Park facility in Eden Prairie, Minn. No practices will be open to the public.
QUOTE TO NOTE: "(He was) always friendly, always open-minded, just very easygoing, just a good-natured person. Obviously, he is dealing with a disease here. Hopefully, he can recover from it in some form or fashion. Really, my heart just goes out to him, and hopefully he is safe, as well as everyone around him." — QB Brad Johnson reacting to the arrest of receiver Koren Robinson.