Obviously in the long run it really doesn't matter who starts opening day.
However, it's always fun to try and guess who it will be.
Most Likely Candidates in my eyes (No Order):
Tommy Hunter
Jason Hammel
Jake Arrieta
Hunter and Hammel are the "veterans"
Arrieta has started a home opener before (last season I believe)
Would you consider anyone else?
There is an apparent dark-horse candidate too. Could it be Wei-Yin Chen?

