If you look at them side by side:
http://www.baseball-reference.com/players/h/harenda01.shtml
http://www.baseball-reference.com/players/j/jacksed01.shtml
There is very little to recommend Jackson over Haren. For example, Haren's K vs. BB (normalized to 162) is 185 to 46 (about 4:1) while Jackson is 150 to 77 (barely above 2:1). WHIP 1.181 vs 1.436. ERA 3.66 vs 4.40 ......
You just made my point. With only one more start in 2012, EJax had a lower ERA, lower WHIP, and more strikeouts than Haren (who, yes, had to face the DH in the American League.)
Even Haren recognizes his declining velocity, and has developed new pitches, which he uses extensively.
So why did EJax improve so much in 2012? Part of it may be Nationals' pitching coach,
Steve McCatty, and the experienced manager, Davey Johnson, and great defense. Gio Gonzalez, Stephen Strasburg, Jordan Zimmerman, and Ross Detwiler were a tight group of flamethrowers who probably helped EJax too. Demoting Lannan to AAA created a complete rotation that offered no letdown when it came to velocity. We now lose that with Haren.
Even in fantasy baseball, you're interested in what someone will do in
2013, not what they did between 2007-2011. I do think that the factors here that improved EJAx in 2012 will help Haren too, who is always looking to improve.
WHAT is McCatty holding? It does not look like a bat.