The current mandate on free agency was designed to make it easier on teams in retaining assets. But in recent seasons we've seen that its negotiations with restricted free agents, players without options elsewhere, that have not just become the most difficult, but present the most critical decisions NHL GMs will make.

None are trickier than talks with defenseman, who typically require the seven-year window allotted before unrestricted free agency to realize NHL potential.

Here's a list of defensemen who must either work out a contract with their parent clubs this summer, or hope for the rare instance of a big-money offer sheet:

Hampus Lindholm, Anaheim Ducks

Though Lindholm contributed nine points at even strength last season, the 22-year-old is truly invaluable under that condition. He's a premier possession defender who's versatile in that he can both facilitate an attack and lock down defensively with his skating and puck skills. With the Ducks working on a self-imposed budget, Lindholm is nothing if not an obvious offer-sheet candidate.

Seth Jones, Columbus Blue Jackets

From shelter to franchise defenseman in a single transaction, Jones' learning program was accelerated last season, and in the process he proved he has the makings of a true No. 1 defender. Jones logged almost 25 minutes a night after being traded from Nashville to Columbus at the midway point of the year, and with Ryan Murphy, formed the makings of something novel to the Blue Jackets: A shutdown pair.

Tyson Barrie, Colorado Avalanche

Barrie is one of seven defensemen to reach 100 points over the last two seasons, and only Erik Karlsson, Brent Burns, Roman Josi, and P.K. Subban have tallied more at even strength. He's hardly dominant in a defensive sense, but it's on the Avalanche to surround that type of talent on the back end with capable help and an efficient strategy.

Jacob Trouba, Winnipeg Jets

With three years experience, Trouba, a big, strong-skating throwback defender with an offensive aspect to his game, has shown signs he will develop into a dominant player. Problem is, all accounts suggest he wants to be compensated as such now, and he's not there yet. He and the Jets just have not appeared to see eye-to-eye since he broke in, and they will have to agree on something for this partnership to continue.

Rasmus Ristolainen, Buffalo Sabres

After being exposed two years back, wearing the label as one of the league's worst statistical defenders as he took big minutes for a club with no intention on winning, Ristolainen took a massive step forward this season. Just four blue-liners logged more ice time league-wide, and with that, he provided more single-season points among Sabres defensemen this decade. Similar to Lindholm and Jones, it behooves Ristolainen's employer to maximize term on a deal this summer.

Matt Dumba, Minnesota Wild

He boasted solid possession and efficiency numbers, and, with a heavy shot on solid puck skills, Dumba has more offense in his game that he's shown. (He ranked 58th among defenders in even-strength scoring rate). The Wild's corps will remain in flux until the expansion draft - it'll be interesting to see how they handle the former No. 7 overall selection.

Cody Ceci, Ottawa Senators

Ottawa's negotiations with the 22-year-old Ceci are critical; this is a team with upwards of $57 million invested in three pieces on the back end over the next three seasons. A long-term contract at a reasonable price point might be the most club-friendly option, but if Ceci's development stalls, the Senators run the risk of being severely hamstrung by big-money investments on their blue line.

Torey Krug, Boston Bruins

Boston's lone puck mover on the back end, and really the only one they can lean on for a little offense, Krug, and his distinctive tool set, is of considerable importance to the club.

Danny DeKeyser, Detroit Red Wings

Dekeyser posted career-worst offensive numbers as well as advanced stats, but had arguably the best of his three seasons as a pro last year. He took on a greater defensive role and steadily improved as the season went on - isolated development critical to a unit that has depreciating assets.

Michael Stone, Arizona Coyotes

The Coyotes have a pair of fascinating restricted contract cases with Connor Murphy also in need of a new deal, but the emerging offensive talent in Stone adds a wrinkle exclusive to the fifth-year man. Stone has a booming shot and the fourth-most assists among those previously listed, but is still working toward being a consistently-sound defender.

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