They were losing by three runs, so Williams, Payamps and Uribe were likely out of the equation.
Milner (the only other lefty) had already pitched, as had Wilson.
With Megill on paternity leave and Teheran throwing three innings on Sunday, that left Chafin, Bukauskas or Rea as the options to come in and face the lefty Nootbar in that situation.
Chafin allowed a walk and a single, but also recorded two outs and allowed zero runs in his appearance.
Craig keeps going to him because he is on the roster and someone has to pitch low leverage innings to save the good relievers for higher leverage situations.