Though I have never used Hershberger, I can definitely vouch for the possibility of losing May for 20-30 games. I have played in Y56 many times and used him frequently, and he has missed 30 for me....
I must also jump in on the Oliva/Brett talk... With Brett's injury risk and the amount of times that Oliva will be useless in games in your own division, I would consider adding Selkirk or Ott to play right. Selkirk can get hurt, not Ott. And a Walling/xxxx platoon would work well.
Personally, I like Selkirk a lot, but he's a risk. Of course, if you move Brett, it's a wash.