Sean Lee screamed in frustration as he was escorted off the field Tuesday.

In the team’s first organized team activity, the veteran middle linebacker hurt his left knee when he collided with rookie right guard Zack Martin and fell to the turf after trying to defend a screen pass.

Trainers immediately went to aid Lee as his teammates turned silent upon realizing what had happened. Lee will visit doctors in the afternoon to determine the extent of his injury. After the practice, Garrett seemed troubled by the fact Lee sustained enough contact that he landed on the turf.

“We keep preaching, ‘Stay off the ground. Stay off the ground,’” Garrett said. “That’s a really important thing. Injuries happen when guys hit the ground.”

Lee can attest to that.

The fifth-year veteran has missed 18 games in his career and has yet to play a full season. Since being drafted in 2010, Lee has been sidelined because of torn ligaments in his right big toe, a dislocated wrist and pulled hamstrings.