Jason Bay the designated hitter

There has been talk that Jason Bay has come back to the Red Sox asking if they could resign him. Obviously there are a ton of problems with resigning him, but that isn't what I wanted to discuss here today. There are a few of things to take into account in a move like that. The first is when we discuss designated hitters we are talking about someone who has no need for defensive value so his level of "average" and "replacement" is very different. This has been articulated many times by Tom Tango, but replacement level at DH is actually average. This should make sense as you could find someone with enough of a bat to hit, but often has poor defensive skills. This effect is why the DH has such a large positional adjustment at -17.5. This is why a league average DH is a much better hitter than the league average player. You can also see how poor a defender needs to be before they would be better at DH.