(as of August 15)
Pena is hitting 221/341/452 on the year. Vs lefties, he's hitting 133/255/322 (attrocious). Jeff Baker (concidentally the same number of at bats) is hitting 356/372/500 against lefties.
If we replace Carlos Pena's vs-lefty at bats with Jeff Baker's, you get 274/357/495. That equals a very good hitter from two ok players patched together. Quade certainly didn't make the most of this opportunity.
Disclosures: the obp was estimated but within 1 or 2 points of the actual, the average and slugging were not estimated but directly calculated.
You can't match them up perfectly every time, sometimes Pena will have to face a lefty. However, Quade didn't even try, because he didn't know it was better. This isn't exactly cutting edge technology here either. Instead Baker faced lefties while Barney sat. Barney has no problem with lefties.
While I'm at it I might as well complain that he didn't play Reed Johnson ever at the beginning of the year when he should have been platooning with Fukudome. These things cost wins that might have ended up mattering (We now know that wasn't the case)