The first time that I saw Santiago Casilla pitch, I rushed to one of my fantasy mags. I wanted to know what people were writing about a youngish eighth-inning guy with a mid-90s moving fastball and an unhittable power slider. “Dollar pick,” the magazine said. “Not worth more than late-round, reserve look.”

The first couple months of 2008 proved that forecast wrong. Casilla was dominating with a 0.93 ERA, a K/9 IP of 10.24, a K:BB of 22:4, and an OBA (opponents’ batting average) of .185. Guys were slugging .262 against him, and he’d surrendered a single home run.

Let me explain what I saw when I watched Casilla pitch: his fastball looked like a boulder rolling downhill. It was bouncing, jutting, curving, then painting the outside corner. His slider dropped like a bowling ball. Batters couldn’t touch it; they couldn’t come close to hitting it. I thought for sure that I’d found the next great set-up man, the AL’s Marmol, the next closer-in-waiting.

Then Casilla strained his elbow, and he’s been out for about five weeks.

But Casilla, who’s owned in 4.8 % of ESPN leagues, is someone who, within the span of a day, could go from being a $1 player (depending on your league) to a $20 player. His injury makes me wonder whether his stuff will have lost some of its potency, but if he’s healthy, this is a guy to buy, hold, and hope for the best.

Casilla, who’s twenty-seven, was signed in 2000, out of the Dominican Republic, as an undrafted free agent. Listed at a slim 6′0″, 160, he’s just one of those guys with a golden arm. He went right to rookie ball, where his domination stats started spiking. A 9.51 K/9 IP in his first season of professional baseball. That was followed by totals of 10.07, 11.68, 14.7, 16, then, in 2005, 16.2. He’s settled at around 10, but I offer those low-minors numbers to give you a sense of his potential.

Casilla’s career minor league totals: 2.94 ERA, 1.18 WHIP, 0.51 HR/ 9 IP, 11.26 K/9 IP, 7.27 hits/9 IP

Casilla wore down toward the end of ‘07, but going forward his arm should be fresh. Huston Street has had his share of elbow strains and arm/shoulder injuries, and the A’s have often turned to scrubs to serve in the ninth inning. Should Casilla prove to be healthy, and should Street go down in the future, the closer’s job is Santiago’s to lose. His stuff is just too good; his walk total is just too low.