It is a good problem to have, namely more starting-calibre talent than starting spots, and this is the problem that the Sounders already appear to have in their crowded midfield for 2012.
With Mauro Rosales, Alvaro Fernandez, and Osvaldo Alonso all seeming to be virtual locks in any "best XI", Lamar Neagle, Erik Friberg, and Brad Evans all having proven themselves capable of starting a match, and Steve Zakuani looking to recover from his horrific injury last April and re-join the fray, it becomes pretty clear that the 2012 Sounders already have a logjam of talent at midfield.
Less is more
In a similar vein to the way that a team moving to a "back three" often really means they are going more defensive, teams going to three midfielders are often looking to own the midfield. In a "three man" midfield the three nominal midfielders usually fill roles, not so much positions. The three roles can be described differently: "creator", "passer" , and "holder", or "destroyer", "playmaker", and "runner", the idea is that the three players need to fulfill complimentary if not slightly overlapping roles.
Not unlike the manner in which a more centrally located "back three" needs to be reinforced by wingbacks - nominally midfielders who are little different in skill set and role from fullbacks - the somewhat amorphous central three need to be reinforced in wide areas. In this case, the reinforcements can come by way of fullbacks pushing up or wingers falling back.
With marauding fullbacks special emphasis is placed on the holder/destroyer to fall back as an auxiliary center back. With dropping wingers, special emphasis is placed on the creator/runner to provide support to the center forward or "lone striker". Combine the two, and play with what has come to be called the "false 9" in contemporary parlance and you are playing what the Dutch really had in mind when they "invented" "Total Football."
No need for symmetry.
If I've said it once, I've said it dozens of times: in soccer-football formations symmetry is actually pretty rare. More often that not, one fullback is more marauding, on outside mid may be more of a forward, central mids may prefer to roam to one side over the other.
How this translates to a "4-3-3" as described above is that one side may be more attacking - with central players shifting to compensate defensively - or even a combining a more attacking fullback with more defensive winger on one side, with the opposite on the other.
But I'll come back to this in a bit...
So what about the Sounders?
It seems pretty natural for Zakuani and Rosales to take up the winger roles, Alonso was born to play the destroyer role, and Fernandez comes to mind as a natural for the passer role. It gets a little vague when trying to determine the other role. A "playmaker" can be lined up deeper (regista) or higher up the pitch (trequartista or CAM). Or, he can even be a 2nd striker or wide midfielder in one of the many versions of the 4-4-2. At this point, it might be best to list the above players in their most favorable roles and relative positions.
Mauro Rosales: creator/playmaker; wide moving central
Alvaro Fernandez: passer/playmaker; wide or sometimes central
Osvaldo Alonso: holder/destroyer; central
Lamar Neagle: runner: wide
Erik Friberg: passer/creator; central or sometimes wide
Brad Evans: holder/passer; central or sometimes wide
Steve Zakuani; runner/creator; wide moving central
Making this fit with Fredy Montero
As we know, it seems like the only only downside to the Sounders running a 4-3-3 is that Montero may not be in his best role as a "false 9". He is certainly capable as a "2nd striker/trequartista, and that, by necessity, requires another forward. It is certainly possible to make this bold step, but probably also unlikely; and for the sake of relative brevity, one I will not pursue here.
So with only 4 midfield spots available, it may be time to revisit the symmetry issue - or lack thereof. Let's just start off by assuming that "el Corazon" will take up the holder/destroyer spot and go from there.
The most compelling case for Mauro Rosales to be included in the lineup as much as possible is the hole left in the attack with his absence. He fulfilled his creator/playmaker role from the right mid/wing spot to great affect last season, and there is no reason to believe he won't do it again. Fernandez's play as a passer/playmaker from the wide left position last season begs for his inclusion as often as possible this next season, but in doing so it also looks to take time from Zakuani and even Neagle. In Zakuani's absence, Rosales took on more of a "runner" roll last season, but better to include a true runner in Zakuani. Assuming he can recover to the point where he can use speed as a true weapon, of course - but one certainly most hope for this. This forces Fernandez central into a central role, in that we have never really seen him play in first-team League action.
But remember what I said about symmetry. There doesn't seem any reason to believe that Zakuani can't start wide and naturally drift to the center, and Fernandez start central and naturally drift wide. The risk is that this leaves Alonso with too much to do in the middle, but given his abilities it may be one worth taking.
With the Sounders blank slate at fullback, the trick may be to find fullbacks to compliment the midfield's changing shape. A more advanced right back could fill the space behind Rosales, while the left back could be more defensive and the three remaining defenders could shift slightly right to form a back three, allowing Alonso to stay in the midfield (and not drop deep as a modern-day "sweeper). The shift could look something like this
Zakuani on the left as more of a winger trending to striker than midfielder, Fernandez as a left-oriented passer, and Montero as a trequartista and/or Rosales trending more central to complete the midfield trio. The right back pushes high as an outside, leaving a back three when .on the attack.
This leaves out Evans/Friberg, which even I admit is unlikely. Evans is on the bench if a switch to more of a second holder is necessary, with Zakuani or Fernandez coming off- depending on what is necessary, or even Rosales and someone switching to the right. It is unclear to me how Friberg fits into this, and Neagle and Fuctio would have to be satisfied with "energy off the bench" roles at outside mide/winger or center forward/2nd striker, respectively.
This is just one idea of many, and it must be stressed that the key to understanding this is the inherent fluidity and overlap of roles and positions that is the legacy of the "Total Football" contribution to global understanding of the game. In all but the most rigid of systems, players have not just one primary role and/or position, but even one (or more!) secondary role and/or position, and the key to modern management may very well be keeping track of how these seemingly infinite combinations fit together.