Michael Sheen said in a recent interview that Sarah will be back in Masters of Sex this season, perhaps even in a scene with him:
DL: Sarah was previously on Masters–will she make a reappearance this season?
MS: Yes. She’s in the next episode, so, she’ll be back. We see a bit more about what’s going on between her and Betsy (Brandt, who plays Barbara Sanderson), on their relationship. I don’t want to give anything away, but yes, we check back in with them.
DL: How much fun is it having Sarah on set with you?
MS: Up to this point, we haven’t done any scenes together, so we’ve never been on set together, unfortunately. But that might change this season!