The door was open but the Rangers stumbled and fell, and so the door was slammed shut on them. Instead of preparing for a Game 7 in Ottawa against the Cinderella Senators on Thursday, they’ll gather for exit interviews and a season-ending meeting with the press.

To be sure, few observers harbored serious thoughts about the Blueshirts knocking out Pittsburgh or Washington in the Eastern Conference Final, but climbing to that level after bowing out in a first-round, five-gamer to the Penguins last spring would have been a noteworthy accomplishment.

However, that wouldn’t have masked the fact that general manager Jeff Gorton has an unenviable summer ahead, the second offseason in which to keep building toward a championship team, because that’s what has to be the end game for the Blueshirts, whose last Stanley Cup title was 23 years ago.

Above all, Gorton can no longer fudge on the frayed blue line. It needs an overhaul. The bar’s been raised. In the East, teams are going to get better. It’s like an arms race. You can’t stand still.

The Rangers can’t instantly turn into the Predators, who have the strongest defense group in the league. But they simply must have a top righthanded defenseman to join Ryan McDonagh, while rookie Brady Skjei moves up to the second pair next season.

Candidates? Unrestricted free agent defenseman Kevin Shattenkirk, who won’t cost them a player, but will shrink the wallet and the cap space. Shattenkirk, 28, will likely command $6 million plus over six or seven years. He’ll help the power play, but longer-term, the dollars for an over-30 defenseman present a risk. If he’ll take $5 million per for five years to play for the team he loved growing up, that’s worth considering, even if it’s a fairy tale.

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Winnipeg’s Jacob Trouba? Size, snarl and speed. He’s 23 and just what the doctor ordered. He’s going to get paid big-time after next the ’17-18 season, when he’ll earn $3.5 million. The problem is, the Jets want a similar defenseman back, and the Rangers don’t have one to offer. And the Jets are loaded with forwards.

Which leaves, in my mind, Carolina’s Justin Faulk, and there might be a match. The Hurricanes have a stable of young defensemen, starting with Noah Hanifin, Jaccob Slavin, Brett Pesce, Ryan Murphy and could use some forwards. Faulk is 25, has 33 goals and 74 points in his last 139 games, and a reasonable $4.8 million cap hit for the next three years. Gorton might have to ship out Kevin Hayes or J.T. Miller and a prospect (Carolina has a handful of picks) to Raleigh, but as the saying goes, you need to give up something to get something.

The Anaheim Ducks have a defensive surplus and some older forwards with expensive long-term contracts (Ryan Getzlaf, Ryan Kesler, Corey Perry) and could use some younger ones. Josh Manson, 25, and Brandon Montour, 23, aren’t stars, but would look good in blue.

To accomplish the mission, the Rangers need to clear cap space not only for least one incoming blueliner, but to pay restricted free agents Mika Zibanejad, Jesper Fast, and Oscar Lindberg.

If aging warrior Dan Girardi, 33, is bought out of the final three years ($5.5 million per) of his contract, he would still cost $2.6 million against the cap next season. To move veteran Kevin Klein in a trade for a draft pick, for example, the Blueshirts might have to retain some of his $2.9 million salary due next season.

The X-factor this summer is the Vegas Golden Knights, who are compiling a team from scratch. Would general manager George McPhee, who likely won’t be able to get top six forwards or top four defensemen in the June expansion draft, consider Rick Nash, with a $7.8 million cap hit in the final year of his contract, if the Rangers retained some salary and offered some draft picks and/or prospects as a sweetener?

Other moving parts? More than space allows at the moment. But somehow, by September, the Rangers need to restructure their defense.