Being a fill-in scorekeeper, I think that if every pitcher was exactly or nearly exactly the same, the 1st relief pitcher would be the guy I would give the win to. But this one is not an easy one... normally in a weird sitch like this, SOMEONE pitches 3-4 innings or so, and is the obvious deserver of the win.
All that is sure here is ... it can't be the starter, and it is very unlikely to be the last pitcher.
How many high schools out there have 9 pitchers capable of shutting down the opponent?
