Wow interesting trade.
In my opinion you should basically never trade a starter for a reliever, but this may be the case where it's worth it.
Tommy Hanson is a decent SP, but:
1. He can't stay healthy. Only 130 innings in 2011, and an injury somewhere around July in 2012 (in the 2nd half he took a hammering).
2. His bad HR/FB of 2012 is not an outlier, it has a twin brother in 2011.
3. He doesn't have the large helpings of GBs (45%+) that elite SPs have, so he has to rely on his K/BB to be north of 3 to be above-average.
I can't believe I'm saying this, but I think the Braves will get more value from this trade.
Walden is under team control for one season more, and if Hanson really is "2012 Hanson" or continues to be a 13% HR/FB pitcher, the Braves are making out like bandits.