Why it will happen: Heading into training camp, it doesn't appear the Steelers have great depth at outside linebacker. With the starters firmly in place - Jason Worilds and Jarvis Jones - Chris Carter and Arthur Moats appear to be the team's top reserves. Moats' can be seen as the versatile linebacker, capable of playing either inside or outside, so an injury to an inside linebacker can be seen in terms of the depth chart as the same as an injury to the outside linebackers. That leaves just Carter with any relevant game experience, and Carter has not shown over three previous seasons he's capable of starting for an extended period of time.
Harrison won't likely fall back into his 16-sack form, but considering his strength and experience, he could serve very well in running situations to help anchor down the outside edge. All three of the Steelers' AFC North opponents have made moves this offseason strongly suggesting they will all look to run the ball heavily this year.
The Steelers fell to a shocking 21st overall mark against he run in 2013, much of which on eight carries that resulted in net gains of 381 yards. Without those big plays, the Steelers' run defense would have finished fourth overall against the run. Still, if teams find a weakness, which they very well may do, considering their right defensive end position in their base defense is still up in the air, they're going to exploit it.
Harrison's presence against that edge may deter them from trying it. If he could bring that kind of impact, the Steelers, a team that clearly wants to be competitive in 2014, may have to make the move for the sake of being the best team they can be.
Why it won't happen: The Steelers can wait through camp and even into the season to see if anything may happen in terms of injuries. They wouldn't have to jump on Harrison now - it's not as if suitors are lining up at his feet like so many battered quarterbacks.
It's difficult for a defense to continue working to get younger if they're adding 36-year-old players who left the roster for a year. Harrison played out of position in Cincinnati last year, and it's not as if he will have forgotten how to play in Dick LeBeau's defense, but adding an aged player brings on a proportionate amount of injury risk in itself, so no net gain may be realized.
The Steelers will bring in loads of outside linebackers to camp, most of whom will be young and looking to catch on, incidentally, much like Harrison did in the early 2000s. A good example is Vic So'oto, a player who has seen time on special teams with Green Bay and could have been on his way to seeing snaps in the team's defense (a 3-4 under Dom Capers) if not for injuries. Bringing in Harrison - assuming both the team and Harrison are on the same page as far as expectations for playing time or even making the team in the first place - may stunt the growth of those younger players like So'oto. Sometimes throwing the young guys in the pool is the better option.
Keys: The development of those younger players must come first. If, in our example, a player like So'oto does have the ability to get on the field. He may not be, snap for snap, better than Harrison, but if he's good enough for the defense to not lose ground when he's there, the team is going to keep with the younger player. That's the nature of the business.
That does not, however, absolve the team of its responsibility to its players and its fan base to put the best team possible in order to win, a very delicate and tricky combination of the present and the future. If the younger guys can run, they're gonna run. If not, Harrison will get a call.