Twenty games into this season, the Toronto Maple Leafs looked like they had emerged as a defensive juggernaut.

Under new coach Craig Berube, they posted the third-best high-danger chance rate in the league, allowing just 8.7 chances against per 60 minutes of even-strength play.

But ever since that stretch, keeping the chances down has been a struggle. In fact, over the next 40 games, the Leafs dropped to 20th in that same stat, allowing 25 percent more high-danger chances against than earlier.

That shift has been a bit confounding to try to analyze. Some of it, perhaps, is due to injuries to forwards earlier in the year, but that doesn’t explain why it’s been such a sticky trend, stretching now into early March.

Increasingly, the blue line has been a key source of the leakage.

The Leafs have really only had to play seven defencemen most of the season. They haven’t had major injuries back there, and the main rotation has been between the bottom three D, Conor Timmins, Simon Benoit and Philippe Myers, who only became a regular in December.

That’s where Toronto is going to need to upgrade, although it’s worth pointing out that the drop in chances against has been happening almost across the board, with everyone save for Chris Tanev affected dramatically.

Of the 213 defencemen who have played a regular shift in the NHL the last 40 games, Tanev ranks 22nd in high-danger chances against.

The other Leafs blueliners rank from respectable — 74th (Myers) and 89th (Jake McCabe) — down to the league’s bottom quartile, 168th to 180th (Benoit, Morgan Rielly and Timmins).

This feels like a big-time problem for a team with championship aspirations.

High-danger chances against

After an ugly shootout loss against the San Jose Sharks on Monday, the Leafs sit fifth in the NHL standings by points percentage. They’re also atop the Atlantic Division, tied in points with the Florida Panthers after their win over the Tampa Bay Lightning.

They’ve had a good year, but avoiding that 2-3 hole in the division is going to be huge in Round 1, given how much of a war that is likely to be.

And while so much of the focus and talk this season has been on their need to upgrade down the middle, the Leafs are also facing a situation where their blue line doesn’t look good enough to take on the best teams in the league.

Even against the lowly Sharks, Benoit was victimized for a key goal against.

Adding another strong defensive centre would certainly help this team in its own end, as will not playing players like Ryan Reaves and getting back defensive forward Calle Järnkrok, who finally made his debut Monday on the fourth line.

But what the Leafs don’t appear to need right now is a big difference-maker on offence. After a slow start to the season, Toronto has re-emerged as one of the highest-scoring teams in the league. They’ve scored more than all but three teams since Dec. 1 — or basically since they started to get healthy from a large run of injuries early in the year.

Matthew Knies has emerged as a key threat, producing at a 35-goal pace — something they didn’t have to nearly that level a year ago at the net front. And Bobby McMann hasn’t been far behind, scoring at a 27-goal pace in a nice breakout campaign.

Between that level of offence and great goaltending, the Leafs been able to win a ton of games.

I realize offence has been a significant problem in the postseason for the Leafs, but the reality is a lot of the issue has come from high in the lineup. And even if the Leafs bring in someone like Brayden Schenn to play 3C — which feels like a long shot right now — they’re going to need better offensive performances from their stars to go deep.

Adding another depth forward doesn’t really change that equation. Someone playing with Max Domi and/or Nick Robertson down the lineup can’t really be counted on to fill that void if Auston Matthews, Mitch Marner and William Nylander aren’t point-per-game players when it counts.

What stands out to me as the most concerning thing on the roster right now isn’t the offence or the third line. It’s the fact they only have two or three defencemen they can count on night to night.

The problem seems especially egregious on the third pair lately. The Leafs depth defencemen have not only conceded an increasing number of high-danger chances, as mentioned above, but they’ve done so while facing fairly easy minutes, with McCabe-Tanev shouldering so many of the difficult matchups.