Left back is an incredibly difficult position to fill. Look around Major League Soccer -- or even around the world -- and it will quickly become apparent that left backs are a valuable commodity, and that great left backs are a true rarity. "We checked probably 50 players and that's not even all," Union Sporting Director Ernst Tanner said of the club's search for Kai Wager, who was signed today from Wurzburger Kickers, to compete with Matt Real and Fabinho for a spot on the left side of defense. "And all the left backs we have been checking, they have flaws. It is an extremely difficult position to get."

It's also an extremely difficult position to play. Modern fullbacks patrol the entire length of the pitch, pushing past the midfield in attack and dropping deep to protect the top of the box defensively. The physical demands are enormous, but the positional understanding -- and, thus, the mental challenges of the role -- are even tougher. 

One of the aspects of the Union's fullback play that requires both cognitive work and physicality is the aggressiveness with which they must play. Aggressiveness doesn't mean simply bounding into tackles though; it's a mental thing. Accepting the space behind you when you go forward -- remaining a front-foot, or forward-first, defender -- is difficult because humans tend to be highly aware of risks in their environment. Empty space between you and your goal for a defender? That's risk. 

The willingness to remain aggressive despite that space is key for Philadelphia's style of play, and it's something that the club preaches in its academy from an early age. Kai Wagner is expected to remain aggressive, which will allow him to use his massive physical tools to pin back the opposition and help the Union play in their attacking end. 

And those tools are big ones: "He's very dynamic, he's very physical. He can cover a lot of ground and he's quick," Tanner said of Wagner. "You have somebody who is going in both directions very quick and they are not so many players capable of doing this." 
It's clear, then, that the Union want a stable of fullbacks that are capable of patrolling the entire wing, from front to back. The complex, high-pressing system they are likely to use begins with control of the center and asks fullbacks to provide width in the right areas at the right times. That right-place-right-time idea is one that takes time to develop, and both Tanner and Jim Curtin will be looking for Wagner to show growth during the early stages of the season. Luckily the club also has a high-ceiling prospect in Matt Real, and until the U-20 World Cup begins, Real will be pushing hard for a starting role after showing well down the stretch with Bethlehem Steel FC last year and captaining the US U-20s in World Cup qualifying. 

Perhaps it hasn't happened quite as quickly as Tanner and the Union wanted, but now the roster is filling out with exactly the types of players the new Sporting Director said he would aim for when he arrived back in late 2018. Young players with a strong mentality and a lot of room to quickly develop into bigger contributors. "With Kai," Tanner concluded, "We have an upcoming player who played on the lower professional levels. But he is on his way to a higher professional league and that's exactly the chance we want to give him here."