"It's almost unexplainable," said Cain of his reaction to the news he received at about 9 a.m. PT Friday.
"I was out to get something done to my truck, but I had to cancel that and now we're here."
Ranked No. 8 in MLB.com's Top 50 Prospects on Aug. 8 and No. 13 in Baseball America
's "Midseason Snapshot" in July, the 6-foot-3, 230-pounder has been on the fast track to the big leagues since being drafted by the Giants in the first round of the 2002 First-Year Player Draft.
"Before I even saw him, I was alerted to the fact that this guy can really throw," said Giants manager Felipe Alou. "He's a very aggressive young guy. And then when I saw the guy throw, I knew that he was going to be here."
A year ago, Cain tore through Class A and Double-A with a record of 13-5 and a 2.67 ERA. He led all Giants Minor Leaguers with 161 strikeouts and his ERA and wins total were tops in the system as well. Following the season, he was named the organizational Player of the Year by USA Today
Beginning this season in Triple-A, he was 10-5 with a 4.39 ERA before Monday's promotion. Cain allowed 118 hits in 145 2/3 innings and his 176 strikeouts were 43 more than the next most in the Pacific Coast League.
"I feel great," he said. "This time last year I felt a little tired, but I feel really good right now. I think conditioning we've kept up with a lot more and I feel good."
If there was any doubt that the Giants considered this move a big deal, general manager Brian Sabean made himself available to the media before the game, something he doesn't usually do for transactions.
"Cain has been very impressive, especially in his last start, and the timing right now works out better than it has in the past," said Sabean.
"It signals that we're making an effort to win games. The bottom line is our young guys are ready in our estimation."
When asked what makes Cain such a special prospect, Sabean rattled off several different qualities, including a strong mound presence and an overall maturation level that is uncommon for players of his age.
"It will be a good opportunity for him to get baptized, especially for a young kid at his age.
"We know that it will be a growth process and it is better to do it now than in Spring Training. Whatever he does, it will a plus for him next year."
With Cain's insertion into the rotation, the Giants now have four starters that have not celebrated their 26th birthday. He is also the fifth player to make his Major League debut with the team this year and one of six rookies on the Giants' current roster.