If he goes on the DL, which is likely, who is going to step up and pitch in his spot? From the A's website:
"Macha said Yabu is a strong possibility after the way he pitched Friday."
I agree it should be Yabu. He's a veteran (though technically a rookie) who will be able to handle the pressure of starting for a team that's not performing well. I think it might be a disastrous mistake to force Rheinecker or Serrano into this tough situation.