DeCastro went down with a knee injury early in the Steelers' third preseason game, and was placed on the injured reserve-recall list, meaning he can re-join the team after Week 8.
He doesn't have to, though. The team has three weeks before he either must return to the 53-man roster (he's currently not counting against it) or be placed on injured reserve for the remainder of the season. That three-week time period does not start until he practices fully.
It's not even out of the question he doesn't return at all.
Obviously, the long-term health of DeCastro is the top priority, and if he isn't ready, he shouldn't return. Ramon Foster has played well enough to think the Steelers can continue to use him until DeCastro is ready to start and play a full game.
If he began practice at his first opportunity next week, it's unlikely he'd play in the Steelers' Week 9 game at the Giants. Giving a rookie who hasn't been on the field for live practice in two months a start probably is not in the team's best interest. Serving as Foster's back-up for at least one week isn't a bad idea, neither is keeping him on the IR-recall list for another week, and keeping the extra eligible player.
If he practices well, shows he absorbed the game plan in preparation for Week 9, maybe give him the start on Monday Night Football in Week 10 against Kansas City in Pittsburgh.