Johnson collided with a defender reaching for a pass in a one-on-one drill and fell to the ground.
Giants coach Tom Coughlin said the initial exam revealed an injury to Johnson's medial collateral ligament.
No prognosis was immediately given, but it was feared that he could miss several weeks. Fifth-round draft choice Kevin Boss is already sidelined with a shoulder contusion suffered in the Carolina game.
Johnson, who saw brief action in the last two games of the 2006 season, had impressed Coughlin with his work here.
“He had a good camp and he had a nice spring and he started to make his move and really has done a good job with his route running,” Coughlin said. “He’s been a guy that’s been very dependable on how his actions would be depending on coverage. It’s really been specific for the quarterbacks.
And any time you have a young player, if you could have it the way you want to, they would have more practices, rather than fewer, and so he’s going to obviously have to deal with this issue and it’s going to take him off the field, which will put him behind. There’s no way around it.”