Last season, after Wily Mo Painful collapsed and Manny Acta tried everyone else from Paul Lo Duca to Felipe Lopez to Rob Mackowiak to Kory Casto to Ryan Langerhans (oh yes, all these guys started in LF for the 2008 Scats), he finally handed Willie Harris the every day job on July 25.
Super Willie took the job and never looked back. With a .251 / .344 / .417 batting line in 424 plate appearances and a 30.8 UZR/150 in 562 innings in left field (meaning his fielding prevented runs at a rate of 30.8 per 150 games, compared to an average fielder), Harris led the team with 3.2 wins above replacement (WAR). Despite sitting for most of the first half of the season, he was tied with Pat Burrell for sixth among National League left-fielders in WAR.
This year, the 2008 HR total is looking like a fluke (his 13 homers last season were almost double his career total to that point), but his overall offense is almost as good at .228 / .350 / .386. And with regular playing time he can be expected to do even better.
And his real value of course is with the glove. He isn't quite as far above average in center as he is in left, and with a small sample size the advanced metrics are a little all over the place. Still, for his career he's a +2.3 UZR/150 CF, and much like the situation last year in left, average centerfield defense would be a godsend compared to the alternative.
As I discussed last week, Elijah Dukes has shown that he's every bit as bad in center as Lastings Milledge--he's sitting on a disastrous -23.7 UZR/150 for the year. But he's a career +11.3 UZR/150 right fielder.
Of course the cost of installing Willie in center and moving Dukes to right is that you have to send Willingham and his .246 / .378 / .516 bat to the bench. Based on his career numbers he's not going to hit that well for the rest of the season, but it's a big drop-off from Willingham's bat to Willie's.
Of course you also get Hammer's below average glove out of the line-up. He's a -4.7 UZR/150 LF for his career, and he'd never played RF at all until last week.
Let's try to quantify all these trade-offs. ZIPs expects Willingham to hit .255 / .357 / .463 for the rest of the season. Harris's projection is .254 / .342 / .386. Then, we gain a whopping 35 UZR/150 by moving Dukes from center to right and 7.0 UZR/150 by replacing Willingham's outfield defense with Harris's. Probably that's a little bit of an overestimate of the difference between Dukes's CF and RF defense, but it's also probably and underestimate of the difference between Willie and Hammer.
Punch all that into my wins above replacement calculator, and we find that the Harris/Dukes combo is worth 2.97 WAR over half a season, compared to just 1.91 for the Dukes/Willingham combo. That's more than one additional win over half a season--a significant improvement that most fans wouldn't expect to see from swapping Willingham for Harris.
Plus, all the gain is on the fielding side, and with our young pitchers coming in, we ought to be prioritizing helping them get outs.
And if all that doesn't convince you, watch this and this and this and this and this.