Karl Alzner is the iron man of the Washington Capitals. In every game since October 8, 2010, Alzner has played a vital role: lots of minutes against tough competition, plus killing penalties.

While Alzner is an unspectacular player (the polar opposite of Alex Ovechkin), he had been solid in his role. And with a $2.8-million RFA deal, Alzner had been affordable too.

But there has been a stark change in Alzner’s impact this season, and it puts the Caps in a difficult position as we near the trade deadline and, this summer, free agency.

Alzner’s individual stats don’t tell us much. Alzner is not a scorer. He’s had exactly 500 shots in his career with just 19 goals to show for it. His 3.8 shooting percentage ranks him 100th out of the 113 defensemen who have had at least 500 shots since 2008-09. Offense is not what Alzner brings to the table.

By the same card, I’m not sure we can use opponent goals to measure Alzner’s impact, since they’ve been so scant this season thanks to excellent work by Holtby and Grubauer. So let’s try shot attempts. The RMNB Snapshot series has already documented how Alzner lags behind his fellow defensemen at shot attempts. What I’m looking at today is the effect Alzner has on his teammates.

Micah Blake McCurdy’s hockeyviz.com has a great tool for this. It’s a visualization of WOWY, or with or without you, a report on how many shot attempts belong to a player’s team when he is or is not paired with another player. For example, if a bunch of players see a higher on-ice shot-attempt percentage when they skate with Lars Eller, a WOWY would tell us that.

In Alzner’s WOWY, we see the opposite. Each box is a player, indicated by jersey number. Black is how that player performed with Alzner, red is the player without Alzner. (Blue is Alzner without the player, which we can ignore for now.)  Top-right means better possession, as measured in attempts; bottom-left is worse possession.

The black, with Alzner, is clustered at bottom-left (bad); the red, without Alzner, are in the top-right (good). In short: players are getting buried when they skate with him.

Here’s the same data, except not adjusted for score, in a table. The difference column you can think of as the Alzner tax, the cost you pay for taking shifts with 27.

Everyone pays.

Numbers as of February 6, via stats.hockeyanalysis.com

Alzner has one major mitigating factor in his favor. By any measure (time on ice, shot attempts, expected goals), he faces the toughest competition among the Caps defense.

Numbers as of February 8, via corsica.hockey

Shifts shared with Alzner are tougher than shifts without him, so some diminished possession should be expected. But not this much. The difference for Alex Ovechkin is a solid 51.3 percent away from Alzner and a miserable 43.1 percent with him.

Something is going on here, but Alzner’s possession impact had not always been so bad.

This is new, and it’s hurting the Capitals. I’ve got suspicions on the cause — Alzner has never been the team’s fastest skater — but I don’t have the information or arrogance to try to diagnose it here.

But if I had a hockey analytics department on my payroll with a week before their next game, I’d definitely task them with investigating the Alzner tax. Hopefully the Caps can find a way to lower it before the trade deadline or the playoffs, but they absolutely must make sure it’s accounted for before the team writes Karl Alzner a massive UFA contract this summer.

Headline photo: Amanda Bowen/RMNB

Whole bunch of stats thanks to corsica.hockey, hockeyviz.com, and stats.hockeyanalysis.com