I think with Mix here, Lampard slides into a #8 role in a 4-2-3-1 alongside Jacobson. Or he slides into a RM role in a bunched up 4-4-2.
I think it's pretty certain at this point that we will be playing a 4-4-2 diamond. It would take a lot of signaling to think that Kreis has any plans to move off his preferred formation, and all signs have pointed toward him employing it.
Remember that Kreis' 4-4-2 is extremely fluid. Read his comments HERE if you haven't yet. Whether Lampard plays RCM, CAM, or LCM, it's not going to make too much of a difference. They'll all be around the box and free to make runs through the middle. In terms of starting lineup though, I think Mix will be considered the CAM, with Grabavoy at LCM and Calle at RCM, and Lampard will replace the least effective one when he joins up with the team in July.
An upgrade from Jacobson at CDM, the destroyer position, would go a long way toward making this one of the top midfields in MLS by year's end. Adding De Rossi would probably give us the top spot.