Little more than a year later, he's a second-round pick -- Oakland's first overall -- in the 2006 First-Year Player Draft.
That's what an extra 10 ticks on a radar gun does for a kid. Cahill, a 6-foot-3, 195-pound right-hander, saw his velocity inexplicably jump over the past 12 months, and his draft status jumped along with it.
Primarily a shortstop as a junior, Cahill threw only 17 innings in 2005. But something happened over the winter that even Cahill struggles to explain. His 6-foot-3, 195-pound frame started generating 90-mph fastballs, and by the close of his senior year he was up to 94.
"I don't really know what happened; I guess it's just my body maturing," he said after being selected No. 66 overall out of San Diego's Vista High School. "I've been doing the same kind of workouts I've always done. I didn't even grow that much. I just started throwing a lot harder for some reason."
So hard that he rarely needed to use the rest of his intriguing repertoire -- he's been throwing a knuckle-curve since trying it out on a Wiffle ball at age 10 -- while posting a 1.77 ERA with 99 strikeouts against 13 walks in 58 innings for as Vista's ace.
"I need to get a lot bigger, and I think I need to improve my changeup," he said. "When I had my fastball working good, I could pretty much blow it by everyone, so I didn't really need a changeup."
According to Baseball America, Cahill's draft status skyrocketed with a handful of late-season appearances against top-flight competition.
"Cahill was clocked at 91 in the seventh inning of a 17-strikeout performance against Oceanside High, an outing that was viewed by more than a dozen scouts and at least three crosscheckers," BA.com reports. "There were twice as many scouts in attendance when Vista sent Cahill to the mound against Fallbrook High, which was ranked 19th in the Baseball America/National High School Coaches Association Top 50 and features at least four players who had signed to play in college next season. Cahill didn't disappoint, striking out 13 with a dominant fastball, knuckle-curveball, changeup, slider and pinpoint control."
Suddenly Cahill, who was so under-the-radar as a junior that he committed to non-scholarship Dartmouth over the winter, was a very popular 18-year-old. He said representatives from "10 or 15" teams made trips to his house, and the teams that didn't come to the house called him.
"They were all pretty much the same," he said. "You can't get any kind of read on where they'd pick you."
So Cahill, who carried a 4.6 grade-point average -- on a 4.0 scale -- at Vista thanks to advanced-placement courses, told the scouts that if he wasn't picked fairly early Tuesday, he'd happily pack his bags for the Ivy League come fall.
"I met with [the A's] a couple days ago, and we agreed that I'd sign if I went in the top four rounds," he said. "And I went in the second round, so ... I'm pretty excited to be playing professional baseball this summer."
Mychael Urban is a national writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.