Cornerback and defensive line are the two biggest positions of need for future depth. It's hard to accept an argument to the contrary. They only have one starting cornerback (William Gay) currently signed for 2015, and none signed for 2016. While it's reasonable to expect Cortez Allen to sign an extension sometime between now and the start of the league year in 2015, this is a league that requires three starting-level cornerbacks - two on the outside and one in the slot in some variation of a nickel package.
The defensive line has Cameron Heyward, who will likely have his team option activated by May 3, and is also a candidate for an extension after that. They signed Cam Thomas, a career nose tackle, to a two-year deal and it was said he'll play defensive end. His $2 million cap hit in 2015 suggests he's on a Prove It deal with the Steelers for 2014. The undrafted Brian Arnfelt and the injured Nick Williams are all that sits behind them, unless you include Brett Keisel, who's not even on the roster at the moment.
The kicker here is it's unlikely a defensive lineman worth the 15th pick will be available when the Steelers go on the clock, and while there are five good first-round level cornerbacks in this draft, and it's likely at least four of them will be around when the Steelers are picking, the argument can be made none of them are good enough to justify the 15th pick.
That generates two conclusions. In order for the Steelers to set up a value-based pick (i.e. BPA) in the second round, they may have to go the Position of Need route in the first round. Or, the Steelers do what they never do, take a risky prospect in the first round.
However the quintuplet of corners - Justin Gilbert, Darqueze Dennard, Kyle Fuller, Justin Verrett and Bradley Roby - are sliced up and broken down, it's reasonable to argue they have a combination of skills that would lead to their selection in the first round. Circumstance might push them to the second round, but it's a solid bet all of them are gone in the first 45 picks.
The Steelers may just have to say if they want one of them, they have to take them at 15.
The possibility of a coveted quarterback being on the board at 15 is getting stronger, based on several reports, and that may lead to a trade market surfacing by the time the Steelers are selecting, but the Steelers are selecting in the middle of the first round, making it very difficult for a team to trade back into the first, and there really isn't much of a need for a quarterback among the teams drafting behind them (there would have to be two teams in that market, one wanting to get ahead of the other to get the quarterback they both allegedly want). Depending on what Cleveland does with the fourth pick, they could either be the most likely to take a passer, or the least. It's possible Cincinnati and Kansas City could make that argument, but both of them draft ahead of Cleveland already (the Chiefs at 23, and the Bengals at 24).
All of that leads to simple logic. If the Steelers want a first-round cornerback, they'll have to take one at 15.
Gilbert's insane level of athleticism makes him an appealing option; he's also the least likely to be available at 15. Dennard is a decent enough cornerback, but without the high-end speed coveted in the position, it's understandable why a team wouldn't want to use a pick in the teens on him. Fuller has better athleticism than Dennard, and maybe is a shade below him in terms of tackling, but is a very good all-around cornerback.
Roby put some bad tape on display in 2013, but was considered the top corner in the class before last season. Verrett could be the best cover corner in the draft, but at 5-foot-9, lacks the height normally associated with a top 15 cornerback.
Four of the five have visited with the Steelers, Fuller being the exception, along with several others.
The Steelers can get a good cornerback at 15, even if he's not worth the 15th pick. Without much of a trade market to move down, it seems likely they'll have to decide whether any of the players at their biggest position of need will be worth it, or risk making their top cornerback acquisition in this draft a player who ranks sixth, or worse, at the position.