The New York Islanders are a different team than they were a year ago in pretty much every significant way.

From a personnel standpoint the changes are obvious.

John Tavares is gone after heading to the Toronto Maple Leafs in free agency, leaving second-year sensation Mathew Barzal as the new franchise cornerstone.

The key decision makers in the organization have been changed with big names. Lou Lamoriello replaced Garth Snow in the general manager role, and they have the reigning Stanley Cup winning coach (Barry Trotz) calling the shots behind their bench.

They are even back to playing games at the Nassau Coliseum on a semi-regular basis.

Given that the Islanders have probably exceeded expectations so far this season and are one of the hottest teams in the league entering play on Saturday night (9-2-1 in their past 12 games) those changes are getting a little extra light shined on them.

Perhaps the most stunning turnaround for the Islanders through the first half of the season has been their play defensively, because it is pretty much a night and day difference from where they were a year ago.

The 2017-18 Islanders were one of the worst defensive teams in recent league memory by giving up an almost unheard of 3.57 goals per game. It was a mark that was not only (by far) the worst in the NHL during the 2017-18 season, but was one of the six worst marks over the previous 20 years.

There was no major defensive category where they were not among the worst in the league, if not the worst.

One of the things that has stood out about the Islanders this season is the fact they have gone from being one of the absolute worst defensive teams to — at least as it relates to goals against — one of the best.

Entering Saturday the Islanders are allowing just 2.56 goals per game this season which is the second lowest total in the league, trailing only a Stanley Cup contender in Nashville. The natural reaction to that improvement is to point in the direction of Trotz for his system and the way he has the Islanders playing.

There does appear to be some truth to that.

Some being the big word here because it’s easy to let a narrative run away from you in a situation like this.

First, Trotz is an extremely successful coach whose resume in the NHL speaks for itself. Ultimately, he knows what he’s doing so it’s not totally out of the question to think any team coached by him would show improvement, and there is evidence to suggest there has been improvement. Let’s take a look at three sets of numbers here relating to the Islanders’ defensive performance (shots against, shot attempts against, goals against, scoring chances against, and high-danger scoring chances against) from the first half of this season, the first half of the 2017-18 season, and the full 2017-18 season.

That is definitely better, and in some areas significantly better. Giving up five fewer shots (and a decrease in scoring chances against) per game over the course of a season can really add up. But it’s not going to add up enough to take a team from dead last in the league in goals against to the top-two without some stellar play from the goaltenders.

That is where the real change for the Islanders is this season.

What sunk the Islanders a year ago wasn’t just the fact they were a team that was constantly bleeding shots against. That was a big part of it for sure, but it was also the fact they received some truly horrific goaltending from Jaroslav Halak and Thomas Greiss that was barely above .900 for the season. Put those two things together and, well, you have the worst defensive hockey team the NHL has seen in 20 years.

How much has the goaltending impacted their performance this season? Even if they experienced the same improvement defensively under Trotz and were getting the same performance in net they would have already given up an additional 20 goals this season. An additional 20 goals over 39 games takes their goals against per game average from 2.56 (second best in the league) all the way to 3.10 (20th in the league). You think they are still flirting with a playoff spot in early January with that kind of defensive showing? No chance.

Greiss has had a wonderful bounce back season in a platoon role, while Robin Lehner, who joined the Islanders on a one-year, $1.5 million contract in free agency, has been one of the biggest steals of the season as the other half of that platoon. His .929 save percentage is tops in the NHL among goalies that have appeared in at least 20 games this season.

This is the true difference-maker for the Islanders this season (perhaps with a little influence from new goalie coach Mitch Korn?).

It’s not necessarily a culture change. It’s not that they are better without Tavares (the offense certainly is not). It’s not even so much that Barry Trotz is that much of a better coach than Doug Weight (though, I don’t think anybody would argue that he isn’t an upgrade).

It is that they have made some incremental improvements defensively and have received All-Star level goaltending from two players they probably weren’t expecting it from at the most important position on the ice.

How long they are able to do that will determine where this season goes for the Islanders.

They still give up a concerning number of shots and chances and don’t score enough to make up for it when (or if) the goaltending regresses. Keep in mind the Islanders had almost the exact same record at this point a year ago before an extensive second-half losing streak ruined their shot at the playoffs. If they want to avoid that sort of second half meltdown again they are going to need Lehner and Greiss to keep playing like their most valuable players.

MORE: Your 2018-19 NHL on NBC TV schedule

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Adam Gretz is a writer for Pro Hockey Talk on NBC Sports. Drop him a line at phtblog@nbcsports.com or follow him on Twitter @AGretz.