For anyone who is interested, here
is a pretty good piece from 2011 on the concept of lineup protection. The main example in the article is discussions about whether Jason Bay needed more protection in the Mets lineup.
Here is a tidbit:
Jason Bay could have hit behind Joe Morgan and in front of Lou Gehrig on the Mets; he wasn’t going to mimic his production in Fenway Park after he moved to Citi Field. And, what would have helped him as much as anything in that dream scenario is the amount of outs saved by adding those two players to the lineup and, thus, extra chances afforded to Bay to produce at whatever level he’s going to produce.