He is a veteran, but he injured his back in a helicopter crash in training and never actually put his boots on the ground in combat. So he's more than a pencil pusher because he is in fact a Marine, and a young version like Krasinski is playing might still be in shape enough for like, the scene in S1 when he's at the camp that gets attacked. I'd believe him picking up a weapon and returning fire there. Or the opening or (book version) climax of Patriot Games.

He can handle a weapon and take care of himself if the situation calls for it, but that's not his role.