Dude was playing, I saw him at least once on ST in the second half when kicking off after a TD.
Frankly, he didn't do all that much during pre-season games, and then got injured for a bit. That opened things up for Shields, and really there is no reason to give up on Shields. Bush has been decent this year, so he's stuck as dime.
If I'm not mistaken, Underwood was going to be playing the rest of the year with a shoulder harness similar to what Chillar played with before he went on IR. That might give some indication that injury has also limited the prospects of Underwood making an impact on defense.
Either way, he is still on the team and plays on Special Teams. He'll be a decent prospect who should get some opportunity to show something in the off-season. Provided, of course, he can stay out of trouble.
The NFL: Where Greg Jennings Happens.