The Designated Hitter named in the starting lineup must come to bat at least one time, unless the opposing club changes pitchers. It is not mandatory that a club designate a hitter for the pitcher, but failure to do so prior to the game precludes the use of a Designated Hitter for that game.
Pinch hitters for a Designated Hitter may be used. Any substitute hitter for a Designated Hitter himself becomes a Designated Hitter. A replaced Designated Hitter shall not re-enter the game in any capacity. The Designated Hitter may be used defensively, continuing to bat in the same position in the batting order, but the pitcher must then bat in the place of the substituted defensive player, unless more than one substitution is made, and the manager then must designate their spots in the batting order.
I'm not sure how it works in game, but according to the rules I think the DH in the original lineup has to bat once.
This is amazing. I was too depressed I couldn't figure out how to even look into what happened if you just DH'd for him. Here's to hoping one day off from everything will be enough to have him 100% for his next start. I know with my batters in my full franchise I played with Boston (every inning of every game) just putting someone at DH would actually increase their stamina for the next game.
I noticed the other day in Diamond Dynasty that Sean Doolittle's card lists his second position as "1B" so I'm kind of curious if he can be used as an everyday position player. I guess it must be possible to do something like you're asking about (like putting a good-hitting pitcher as your DH) but somewhere along the line the way the implementation of this was either abandoned or never finished by SDS.