Jamie Goldberg of the Oregonian reports that Heath has undergone an MRI but doesn't specify what's wrong with her knee.
http://www.oregonlive.com/portland-thorns/2014/07/portland_thorns_midfielder_tob_1.html#incart_more_sports
Goldberg mentions that Niemiec and Moros are also out with injuries.
Given Mana Shim's residency in Coach Riley's doghouse I would assume that we'll see Farrelly Sunday. Based on what happened to the Thorns midfield after Heath was subbed off against Chicago on Thursday this is probably not good news.