Lots of things he could be doing other than holding a pity party for himself.
Talk to coaches and QB about what to do when they get the ball back. Looking over the play he fumbled and seeing from what side Gilbert came to swat the football, so he can learn from the play. Watching the defense make plays and get them the ball back and being ready to go on the field and win the game.
The Steelers had 13 offensive plays in the game, after the fumble and Johnson accounted for 1 of them, a reception of 10 yards. Did he check out and give up, or did coaches or Rudolph not send the ball his way because he mentally checked out? I dont know, but he didnt exactly look like a guy that had much fight left in him.