Union’s signing of Kai Wagner shows why Ernst Tanner traded Keegan Rosenberry

The hints have been there for a while now, and they’ve been growing.

Trading Keegan Rosenberry was one. A series of training camp drills with three defenders facing five attackers was another. Then came the 4-4-2 formation that the Union have used in their three preseason scrimmages so far, with the midfield four aligned in a narrow diamond.

On Wednesday, sporting director Ernst Tanner finally blew the lid off. He sees the role of outside back in a different way from how the Union, and much of American soccer, ever have.

“We are not using wingers anymore,” Tanner said. “More important is the function of the left fullback — but it is not the classic left fullback who is more from the defensive side. We are calling a left fullback in a way, in the offensive side, we are calling him a wingback. And on the defensive side, we are calling him an outside back. He needs to play both roles.”

It’s time for Union fans to get used to that.

Most American soccer fans are used to watching teams whose fullbacks overlap with wide midfielders in a flat 4-4-2 or triangular 4-3-3 formation. In Tanner’s playbook, the outside backs are the only wide players on the field.

During Earnie Stewart’s time in charge, the Union played a 4-3-3 that suited Rosenberry very well. When he would go forward, the right winger in front of him — usually C.J. Sapong, Ilsinho or Fafa Picault — could retreat to cover the space Rosenberry left behind. When Rosenberry returned to the back line, the winger would return higher up the field.

Tanner has a different set of sensibilities. His focus is on the center of the field, with two forwards and four midfielders building plays and forcing turnovers. The flanks are left exclusively to the outside backs. They are expected to cover the entire length of the field, getting forward in the attack and getting back fast.

As has been written here before, that playing style has become increasingly common in Europe, from Tanner’s old German club TSG Hoffenheim to English Premier League leaders Liverpool. We’ve also seen it some with the U.S. women’s soccer team, where coach Jill Ellis has converted Crystal Dunn from a midfielder into a wingback.

When the Union went shopping for a left back this winter, they wanted a player who could fit the new mold, not the old one. They believe Kai Wagner qualifies. Manager Jim Curtin described him as a player “that can be responsible offensively for the entire left side, and defensively for the entire left side.”

You can get a sense of Wagner’s abilities in this video:

Rosenberry can shut down opponents one-on-one when they’re running at him, and he’s great at bringing the ball forward to contribute to attacks. But he isn’t as good at tracking back.

That was a big problem in Union’s biggest games of the year: the regular season-ending loss at New York City FC and the playoff loss in the Bronx three days later. Rosenberry got caught upfield too often, and sometimes took inefficient routes when retreating to the back line. Soccer analysts track that just like baseball measures how outfielders chase down fly balls.

One of Rosenberry’s most glaring mistakes led to New York’s second goal of the playoff game. After helping an attacking buildup get the ball down the field, he turned the ball over. City broke the other way, and Rosenberry never came close to catching up.

In that context, it’s not so surprising that Tanner judged Olivier Mbaizo — who’s faster than Rosenberry and four years younger — to be a better fit. When the Colorado Rapids offered $300,000 for Rosenberry, with incentives possibly adding another $100,000, Tanner took the money.

Expect the Union’s tactics to continue evolving as the outside backs become comfortable operating alone. The Union could easily adapt to a 3-5-2 akin to what Tanner helped Hoffenheim install a few years ago. He hired the team’s current coach, Julian Nagelsmann, who is one of Europe’s rising stars.

The Union have the forwards, the central midfielders and the centerbacks to do it — but the system can’t work without the right outside players. That’s the last piece of the puzzle, and Wagner, Mbaizo, Matt Real and Ray Gaddis must help solve it.

“We’re going to demand a lot out of our outside backs,” he said. “That’s where we’ll get most of our width, and we’ll have a midfield that will mostly be responsible centrally … I think we have a good stable on both sides of guys that are physically able to do it and tactically able to do it.”