Essentially all sides in MLS (18, if we're to be precise) generally play with a four-man defense, and it's very much the accepted tactical outlay across world football. As such, modern attacking formations are based heavily on attempting to exploit the vulnerabilities and spaces between the two central defenders and the full backs. Chivas USA, as has been oft-discussed this season, has been playing with a three-man back line under Chelis, which presents some interesting options for Real Salt Lake.
The midfield attack
It becomes a rather simple equation — though not an entirely accurate one — when we talk about the three central defenders facing two strikers: 3v2 would seem to give an advantage to the defense, but not because of pure numbers. It simply allows for more defensive flexibility in covering an attacking midfielder — a Javier Morales, say — when he's playing centrally. In this case, you'd have both strikers covered and a third attacking player, and your wide midfielders are covering the flanks.
This is where it gets a bit tricky: If Javier Morales makes his diagonal runs toward touch on either flank, he opens up different sorts of space that the 3-5-2 might not accommodate readily. If the third central defender pulls off with him toward the channels on either side, a run is opened up for a deeper-lying midfielder. If the defender stays, he's forced to decide between covering for a wider central defender, who might peel off to cover Morales, or to simply cover dangerous spaces at the back. Whatever the defense does to attempt to cover the danger Morales provides, options should open up for midfielders.
The strikers might be more efficiently covered, doubling the importance of midfield attack. Any disruption that can be offered — while avoiding too much opportunity for Chivas USA to counter — will play into RSL's hands. A quick turn could see the defense presented with three, four or even five attacking options. Imagine, for example: Saborio, Findley, Gil, Morales and Grabavoy all moving toward goal, and rather narrowly at that. Quick runs past the defenders could see Findley perfectly poised to cause significant trouble.
The wide areas
Chivas USA are likely to control the wide areas with either wing-backs or wide midfielders patrolling the flanks. If Real Salt Lake surrenders them, as so often we do out of tactical necessity, our fullbacks will need to be actively communicating with the midfielders to handle any danger on the flanks. Obviously enough, this will require a bit more communication across the board, as it would be rather disastrous to allow free runs in the middle from a creative side.