http://www.fangraphs.com/statss.aspx?playerid=978&position=P
Even if you use bWAR, 2.7 and 2.5 is still an above average starter, and you're also ignoring 6.8, 3.3, 1.5, which would probably make him a top 15 starter in all of MLB during that time period. That's a huge asset to give away for just Wily Mo Pena. ERA+ assumes the pitcher is 100% responsible for all earned runs, and doesn't put any weight on innings pitched.
The Red Sox won the World Series in 2007, because they could sweep mistakes like this one under the rug. Traded a #2/3 starter? No problem. Sign a Japanese import for $100 million to fill that hole. RF of the future sucks? Go sign JD Drew for 5/70 to be the RF.
It was a bad trade. I await your cherry-picked rebuttal.