Here's the response from my ticket:
Mcgee wasn't at 100%. He was at 95% with an auto rest setting of 98%. That would be the reason he didn't play. There were other players on the roster that were also below their 'rest' setting. Thanks.
This makes sense, I guess. I always have my rest settings at 98 and have never had this issue before. The bigger question, in my mind, is why the sim chose a SP who started the previous day to DH. If it had to use a pitcher, I had other, more rested, pitchers.
Oh well. I will lower my rest levels and hopefully that will do the trick.