There's only 3 guys in Strikeforce who could fight Silva at middleweight. One trains with Silva, so they won't fight. Other is at light heavy and said he'll only drop down to 185 if he loses at 205. The other being their current middleweight champ who'll probably need a win or two in the UFC before he challenges for the title.
Anderson Silva is not leaving middleweight entirely, he just wants his big money fights. There's still lots of fights left for him at middleweight even before Strikeforce comes over. Alan Belcher, Michael Bisping, Chris Weidman, Tim Boetsch, Brian Stann, rematches with Hendo, and Belfort.
He's got lots of options at 185, he just needs to pick somebody. Chris Weidman has the best chance at 185 to beat him. He's a scary good wrestler with sick striking skills and a jiu-jitsu game that comes out of nowhere. That's the fight that needs to happen.