By Dan Durkin–

(CBS) There’s a natural ebb and flow to the NFL game, punch and counter-punch spar between offenses and defense. Recently, offenses have been the aggressor, dictating to defenses.

Three-wide receiver sets – which are really four-wide receiver sets for teams with athletic tight ends – have forced defenses to play the majority of their defensive snaps over the course of a game (and season) with five or six defensive backs.

Thus, the skill set of playing the safety position has changed drastically.

Traditional box (strong) safeties are nearing extinction. No longer can teams just drop a safety down as an eighth run defender without sacrificing something in pass coverage. The reality is, the skill set of a safety tandem must be interchangeable.

Modern safeties must possess instincts, anticipation and timing as well as football intelligence. They are the quarterback of the defense, responsible for lining up the secondary and getting in any last-second checks. If a check is missed, they must be agile enough to recognize as much and improvise accordingly.

What’s making the evaluation of a safety difficult for NFL scouts is the prevalence of spread sets, packaged reads and the frequency of tunnel/bubble screens in the college game. Thus, players aren’t as frequently in position to make as many plays on the ball, so coverage ability is difficult to gauge. More than ever, scouts have to project by scouting traits.

In the end, this has led to a scarcity of talent at the position league-wide.

The Bears have gone through 18 different starting combinations at safety since the 2009 season. Assuming free agent Chris Conte moves on, they’ll be assured of their 19th come September.

New defensive coordinator Vic Fangio believes in interchangeable traits from his safeties as well. Just a few seasons ago, he had the Pro Bowl pair of Donte Whitner and Dashon Goldson. Both were thumpers, with Goldson being more of the mold of a free safety, as he was frequently used in single-high sets and “robber” techniques.

Both Whitner and Goldson eventually cashed in as free agents, and Fangio replaced them with 2013 first-round draft pick Eric Reid and veteran Antoine Bethea. The 49ers also spent a 2014 first-round draft pick on Jimmy Ward, who was frequently used as a slot corner in the team’s “big nickel” package (three-safety look).

Other than Alabama’s Landon Collins, there are no safety prospects worthy of a first-round pick. Thus, teams with safety needs will be competing for the services of just a handful of quality players in free agency.

Here are some of the top options on the free agent market.

Devin McCourty (5-foot-10, 195 pounds, 27, six-year veteran)

A Patriots’ first-round pick in 2010, McCourty has successfully made the transition from cornerback to free safety, becoming one of the league’s premier single-high defenders. The Patriots elected to use the franchise tag on kicker Stephen Gostkowski instead of McCourty, which makes him the clear-cut top option on the open market.

Along with Darrelle Revis, McCourty played pivotal roles in giving the Patriots’ scheme flexibility on defense to tailor the best  game plan to beat an opponent that week. McCourty possesses the lateral quickness, instincts and ball skills required to excel as a free safety and served as a true eraser over the top when a defender required deep help. He has the ability to quickly drive down from the middle of the field to the hash mark to disrupt passes.

Per ProFootballFocus, McCourty was targeted 22 times and allowed 15 receptions for 176 yards and two touchdowns, while intercepting two passes. He will be the most coveted player on the market and likely sign a deal with an average annual value of $8 million-$10 million.

Rahim Moore (6-foot-1, 195 pounds, 25, five-year veteran)

A Broncos’ second-round pick in 2011, Moore has familiarity with Bears coach John Fox. Moore is infamously known for terribly misjudging a deep ball to Ravens receiver Jacoby Jones in the 2012 AFC divisional round that extended the game into overtime, which the Broncos eventually lost. Despite that blunder (in Moore’s second season as a pro), he’s turned into one of the league’s more reliable free safeties.

Moore’s at his best in deep zone coverage. He’s sound in his drops and deep responsibilities, showing loose hips and the ability to transition and pattern match deep receivers. However, Moore can be slow to diagnose plays. While slightly built, he is a willing player in run support but does miss tackles in the open field. At 25, Moore has the physical traits needed to succeed at free safety, and his best football is ahead of him.

Per ProFootballFocus, Moore was targeted 22 times and allowed 17 receptions for 183 yards and three touchdowns, while intercepting four passes. Given that Moore is an ascending player at a position that is short on talent, he will likely sign a deal in the with an average annual value of $6 million-$8 million.

Da’Norris Searcy (5-foot-11, 207 pounds, 27, five-year veteran)

A Bills’ fourth-round pick in 2011, Searcy excelled last season in a full-time role. Previously, Searcy was used in sub packages as a blitzer off the edge, but after the Bills lost Jairus Byrd in free agency, they moved Aaron Williams into Byrd’s role, and Searcy became the starter at strong safety.

With a stout build, Searcy is comfortable patrolling near the line of scrimmage, showing the ability to work through the trash and make plays as a force or alley player against the run. Searcy gives up length in coverage but has nimble feet to pattern match up the seam and maintain good body position to make plays on the ball once it’s in the air.

Per ProFootballFocus, Searcy was targeted 19 times and allowed eight receptions for 83 yards and no touchdowns, while intercepting three passes. Given Searcy’s ability to be both a factor against the run and contribute in coverage, he will likely sign a deal in the with an average annual value of $6 million-$8 million.

Dan Durkin covers the Bears for CBSChicago.com and is a frequent contributor to 670 The Score. Follow him on Twitter at @djdurkin.