For the seven years of the Seattle Sounders MLS existence, Sigi Schmid has almost exclusively used a 4-4-2 formation. Much of that time, that choice has come down to the fact that Osvaldo Alonso is one of the most offensively and defensively dynamic players in MLS. His skill on both sides allows the Sounders to effectively fill the roles of two players at once, freeing them up to shift other players duties to suit their particular skill sets better.
Players like Alvaro Fernandez, Mauro Rosales, and Chad Barrett have occupied more offensive wide midfield roles, these highly specialized players on the wings who offer far littler on defense than one would expect from a standard 4-4-2 stems from Alonso.
For teams that don't use a player like Alonso, the three-man midfield is a effective solution to retain those skills in the middle and allow the team to field more attacking influence. For a Sounders midfield that retains a core set of players with diverse roles, the three-man midfield may be one of the better tactical solutions moving forward in addition to suiting Alonso if he' still on this team.
Types of 3-man midfields
Like most tactical concepts, there are many different implementations of the same basic philosophy. From the archetype central attacking midfielder and two defensive midfielders to the inverted midfield featuring a deep-lying distributor and two box-to-box midfielders to the modern vogue of the double pivot and false 10, the implementations are as diverse as the players who can make them up.
For this I'm going to focus specifically on 1 type: the inverted midfield. The archetype setup of a CAM with two defensive midfielders is archaic at best and the Sounders currently have a grand total of zero players who fit into that model of system. Implementing that kind of play would require a fundamental re-design of the entire roster and since we know that isn't happening if that's your wish you can kiss it goodbye.
I'm also not going to discuss the double pivot and false 10 because it's spatially identical to what the Sounders already do. While the box-to-box roles are dramatically more aggressive during buildup play than the traditional double pivot and Clint Dempsey plays much higher up the field, rehashing what the Sounders already do isn't a particularly fun, or useful, exercise.
The Inverted 3-man Midfield
The inverted three-man midfield is one of the newer tactical vogues in world soccer in that it's only really been practical in the last decade. The move away from restricted roles likes registas and CAMs has coincided with the rise of more rounded midfielders, capable of playing both deep in defense and pushing into attack. Most effective inverted three-man midfields are built around the strength of one pivot player. For the Sounders that option is shaping up to be something that Sigi Schmid can actually mix and match.
If Alonso remains with the Sounders, he'll easily take the predominant number of minutes in the pivot role as he fills both the defensive and offensive ability of that role to exacting specificity. Around him, the Sounders can mix and match to their hearts' delight opting for attacking ability, playmaking ability, or passing ability using a combination of Erik Friberg, Cristian Roldan, and Andy Rose (assuming he returns to the Sounders).
Without Osvaldo Alonso, the situation becomes more a game to game choice on how the midfield is structured. The key in each setup is how Sigi Schmid uses Erik Friberg. In both cases, it's clear that the Seattle Sounders are missing a dramatic amount of midfield depth and they are going to have to be aggressive at filling those holes this offseason if they move to a three-man midfield.
Erik Friberg as the pivot
The main playmaker in the inverted midfield either play in the pivot, or plays off the pivot and moves between opposition defensive lines. If the playmaker is played in the pivot, more akin to more traditional regista, he has to be paired with defensively two aggressive box-to-box midfielders to provide stability. For the Sounders' part, they have one to none of those with just Rose capable of slotting into that type of role. Roldan is still likely too raw on defense for this type of setup to be the day-in and day-out option, but would easily be a solid rotational piece in this type of setup.
Where Roldan can really shine in this setup through is as the pivot depth behind Friberg. There his read of the game, rather than his physical abilities, are more important defensively as the box-to-box midfielders shoulder most of the defensive load. And on those occasions when the midfield structure is compromised, having a player with speed who's adept at pressing and stopping attacks like Roldan is a far bigger boon than using a slower more deliberate defensive option.
If the Sounders move forward with this type of setup they need to target aggressive box-to-box style midfielders -- in essence guys in the vein of Andy Rose. Finding a solid starting option wouldn't necessarily be a priority for the Sounders, but as far as depth options go MLS free agency is littered with these exact types of players, with Ned Grabavoy and Paulo Nagamura being immediately apparent options.
Erik Friberg as the advanced playmaker
If Erik Friberg is lined up as a more advanced playmaker, the Sounders' needs dramatically shift away from those needed to play him deeper. With less of an emphasis on distribution, the Swede is more tactically malleable. Swapping in Andy Rose shoulders Friberg with the creative duties and more defensive responsibilities, while dropping Cristian Roldan alongside him allows him to operate more akin to a double pivot. In this scenario his partner becomes a game-to-game tactical choice that should allow Sigi Schmid to mix-and-match depending on the oppositions pressing style and defensive arrangement. With just three midfield options available now, the Sounder would need to target another box-to-box style midfielder to add as a depth piece, but that would be a luxury rather than a necessity.
Necessity comes into equation deeper as the Sounders would need a defensive midfielder who can distribute well along with a subsequent depth option. If Andy Rose is in the fold, the Sounders could go in with him as the backup option considering how well he proved himself in that role against the LA Galaxy. With guys like Nathan Sturgis or Brain Carroll available via free agency, there are solid options within MLS that could fit a Sounders team on a budget. But honestly, I'd feel more comfortable if the Sounders brought in a more experienced or technically capable player than either of those two options.