Over the past few years, Fredy Montero's evolution as a player has forced a shift in the Sounders tactics, with the 4-4-2 in its most conventional form discarded in favor of a system that depends upon a creative forward playing in the hole just behind the striker, dictating tempo, springing attacks and testing the keeper from distance.
The true potential of that system was realized in the latter stages of the 2012 season once Montero and Eddie Johnson got comfortable with one another. With Montero now playing in Colombia the Sounders will either need to find someone to fill that role in some capacity or make some kind of tactical change. If Mauro Rosales is Seattle's best option on a like-for-like basis, the latter option may well be preferable.
Rosales is plenty creative, but his style of play just isn't well suited to the physical nature of the middle of the park in MLS and his greatest strengths are largely wasted in a central role. The Sounders looked significantly more dangerous once Steve Zakuani was withdrawn for Sammy Ochoa, and that likely has a great deal more to do with Rosales shifting into his more familiar wide position than any significant contribution from the big striker.