They're both basically average facing left-handed hitters, and at that, they posted very similar platoon splits against lefties in '08 (.247/.289/.389 for Santana, .233/.313/.367 for Parra). The difference in their performances can be attributed almost solely to their respective performances against right-handed batters.
That's right, it's left-handed platoon time. I've been itching to look at lefty/lefty platoon splits for a while, to see what kind of lefty pitchers actually provide a platoon advantage.
This one was simple: I listed the NL 2008 average lefty vs. lefty platoon split, and then listed all left-handed pitchers on NL 2009 rosters that pitched in the MLB in 2008 (no matter the IP. You can raise a sample size issue if you like, but I say that that's the whole point! I mean, we are doing platoon splits after all; that's basically writing 'SAMPLE SIZE' on your forehead in black marker).
So, here we go...
Above Average (26 P; 19 RP)
Daniel Ray Herrera, Cin (RP). .000/.200/.000 (.200) 7.3 IP
Jeff Ridgway, Atl (RP). .133/.133/.200 (.333) 9.7 IP
Michael O’Connor, Was (RP). .167/.167/.167 (.333) 9.0 IP
J.C. Romero, Phi (RP). .102/.193/.153 (.346) 59.0 IP
Arthur Rhodes, Cin (RP). .157/.253/.200 (.453) 35.3 IP
Tim Byrdak, Hou (RP). .135/.222/.247 (.469) 55.3 IP
Sean Burnett, Pit (RP). .171/.238/.276 (.514) 56.7 IP
Scott Schoeneweis, Ari (RP). .178/.243/.277 (.520) 56.7 IP
Oliver Perez, NYM (SP). .158/.250/.271 (.521) 194.0 IP
Billy Wagner, NYM (RP). .220/.283/.244 (.527) 47.0 IP
Scott Olsen, Was (SP). .187/.262/.276 (.538) 201.7 IP
Mike Hinckley, Was (RP). .222/.263/.278 (.541) 13.7 IP
Hong-Chih Kuo, LAD (RP). .202/.216/.340 (.557) 80.0
Mitch Stetter, Mil (RP). .158/.304/.263 (.568) 25.3 IP
Pedro Feliciano, NYM (RP). .210/.280/.295 (.575) 53.3 IP
Franklin Morales, Col (SP). .200/.182/.400 (.582) 25.3 IP
Justin Hampson, SD (RP). .250/.391/.292 (.583) 30.7 IP
Phil Dumatrait, Pit (SP). .206/.351/.238 (.589) 78.7 IP
Alex Hinshaw, SF (RP). .205/.318/.274 (.592) 39.7 IP
Barry Zito, SF (SP). .213/.316/.287 (.603) 180.0 IP
Trever Miller, StL (RP). .209/.305/.308 (.612) 43.3 IP
John Grabow, Pit (RP). .239/.321/.296 (.617) 76.0 IP
Wesley Wright, Hou (RP). .207/.295/.326 (.621) 55.7 IP
J.A. Happ, Phi (SP). .209/.261/.395 (.656) 31.7 IP
Greg Smith, Col (SP). .232/.270/.393 (.663) 190.3 IP
Scott Eyre, Phi (RP). .220/.264/.400 (.664) 25.7 IP
Within 3% of Average (9 P; 5 RP)
R.J. Swindle, Mil (RP). .333/.333/.333 (.667)
Charlie Manning, Stl (RP). .203/.284/.392 (.676)
Johan Santana, NYM (SP). .247/.289/.389 (.678)
Manny Parra, Mil (SP). .233/.313/.367 (.680)
Doug Slaten, Ari (RP). .232/.317/.375 (.692)
Jonathan Sanchez, SF (SP). .235/.287/.424 (.711)
Jaime Garcia, Stl (RP). .250/.400/.313 (.713)
Alan Embree, Col (RP). .232/.304/.415 (.719)
Jamie Moyer, Phi (SP). .240/.321/.400 (.721)
Below Average (35 P; 15 RP)
Jo-Jo Reyes, Atl (SP). .255/.314/.415 (.729)
Jack Taschner, SF (RP). .279/.339/.394 (.733)
Royce Ring, StL (RP). .264/.339/.396 (.735)
Bill Bray, Cin (RP). .260/.360/.384 (.744)
Andrew Miller, Fla (SP). .226/.378/.366 (.744)
Jeremy Affeldt, SF (RP). .269/.301/.444 (.745)
Pat Misch, SF (RP). .281/.309/.438 (.746)
Zach Duke, Pit (SP). .279/.344/.405 (.750)
Tom Gorzelanny, Pit (SP). .261/.373/.391 (.765)
Jeff Francis, Col (SP). .248/.312/.460 (.772)
John Lannan, Was (SP). .259/.317/.460 (.777)
Cole Hamels, Phi (SP). .262/.308/.471 (.779)
Jorge de la Rosa, Col (SP). .289/.353/.455 (.807)
Wandy Rodriguez, Hou (SP). .282/.311/.500 (.811)
Clayton Kershaw, LAD (SP). .250/.337/.475 (.812)
Dan Meyer, Fla (RP). .314/.385/.429 (.813)
Randy Wolf, LAD (SP). .283/.368.447 (.815)
Sean Marshall, ChC (RP). .269/.354/.463 (.817)
Tom Glavine, Atl (SP). .290/.355/.464 (.819)
Doug Davis, Ari (SP). .321/.384/.438 (.822)
Boone Logan, Atl (RP). ,291/.324/.505 (.829)
Eric Stults, LAD (SP). .314/.314/.543 (.857)
Neal Cotts, ChC (RP). .269/.338/.522 (.860)
Renyel Pinto, Fla (RP). .264/.371/.529 (.900)
Ted Lilly, ChC (SP). .307/.386/.542 (.928)
Randy Johnson, SF (SP). .303/.382/.545 (.927)
Mike Hampton, Hou (SP). .339/.369/.559 (.929)
Taylor Tankersley, Fla (RP). .360/.467/.480 (.947)
Scott Elbert, LAD (RP). .417/.500/.500 (1.000)
Jonathon Niese, NYM (SP). .353/.353/.647 (1.000)
Matt Chico, Was (SP). .351/.377/.632 (1.009)
Wade LeBlanc, SD (SP). .318/.423/.636 (1.059)
Mike Gonzalez, Atl (RP). .259/.250/.815 (1.065)
Joe Thatcher, SD (RP). .414/.424/.862 (1.286)
Eric O’Flaherty, Atl (RP). .500/.579/.875 (1.454)
So, the best platoon advantage for left-handed relievers in 2008 -- among prospective NL 2009 relievers -- belong to J.C. Romero, Arthur Rhodes, Tim Byrdak, Sean Burnett, and Scott Schoeneweis. Notably below average lefty v. lefty platoon splits belong to Jeremy Affeldt, Mike Gonzalez, Renyel Pinto, Tayler Tankersley, Neal Cotts, Sean Marshall, and Bill Bray. Managers facing those relievers might think twice about leaving their lefty-bat in the game.
What I found especially interesting is that starting left-handed pitchers fared worse against lefty bats than relievers. Off the top of my head, I'd guess that that occurs simply due to how relievers are used, and also how those starting pitchers approach the game. Some of the more prominent pitchers on the "below average" list throw a notable change up (see Lilly and Hamels), and others rely on either a slider or cutter (ever seen Doug Davis and Randy Johnson in a sentence together other than "sharing the 2008 Diamondbacks' rotation..."?). Both cutter/slider and change up pitchers utilize a major pitch meant for neutralization, which might explain why those pitchers pitch relatively poorly against lefties.
So, be careful with your left-handed relievers, and be careful pinch hitting against them. Platoon advantages can indeed work, if you manipulate them properly, but there are situations in which managers might prefer to leave left-handed bats in against left-handed pitchers.
A pitch-type by pitch-type batting order might help a team to produce more runs, rather than a strict platoon based on handedness (i.e., perhaps batting orders should be constructed to face either curveballers, sinkerballers, change-up artists, or slider pitchers, etc.)