And consider it a positive experience.
"It was great," said Cain of his five-inning, 103-pitch debut, which hinged on two pitches -- a homer to Matt Holliday in the second inning and a double-play grounder in the fourth that brought home the winning run.
"There's a ton of stuff I can learn from it," said Cain, currently the second-youngest player in the Majors, with only 19-year-old Mariners hurler Felix Hernandez taking honors. "I don't think there's anything I can take away from it that's bad. It's kind of unexplainable -- there's so many things I can do to get better and it's nice to know that now."
Cain was so locked into his game plan that when introduced while warming up in the bullpen the crowd of 36,433 gave him a standing ovation and he never even knew it happened.
What will he think about years from now of his effort?
"Probably all of it," he said. "I don't think there should be anything left out. Every single part of it was a great moment for me. Hopefully, I'll save it and cherish it for as long as possible."
Catcher Mike Matheny called it a "shame" the Giants managed only a solo homer by Moises Alou -- his 15th of the season -- to back up the rookie, but said Cain had "electric stuff" and had a wonderful overall outing.
"I thought he did a great job," said Matheny. "Obviously he should be proud of what he's done to get here and then to pitch as well as he did. It's a shame we couldn't put up a win for him.
"He had the kind of stuff you don't see very often -- the velocity, the fluid motion that's very deceptive. The ball jumps out of his hand like he's not putting much effort into it."
Perhaps the most memorable at-bat of the game was Cain facing five-time All-Star and former National League batting champion Todd Helton in the fifth inning. It was a thrilling, 14-pitch, nine-foul-ball duel -- with Cain "winning" it as Helton flied out to left field.
Cain was highly impressed with Helton's batwork.
"I made some really good pitches at the end and he did a great job," said Cain. "He has tremendous hands and just missed that pitch at the end. I went with Matheny. We went in and out, sliders, curveball, fastballs. It all worked out."
Even Colorado manager Clint Hurdle was enthralled by that confrontation.
"You only get one chance to make a first impression and he made a pretty nice one," he said. "He put a cherry on top at the end with that confrontation with Helton. He threw the kitchen sink at him and he got him out. That was solid stuff."
Holliday said he enjoyed watching that battle, feeling Helton deserved a hit or a walk at least.
"He can do that," said Holliday of Helton. "Todd is a freak like that. He can foul off almost any pitch, especially when he gets into one of those grooves."
Regarding the hype and hoopla surrounding his debut -- Cain is the youngest San Francisco starter to debut since Mark Grant, who made his first appearance in 1984 -- the native of Memphis, Tenn., was glad it was over.
Cain did have nine family members and three high-school coaches cheering him on, and none seemed upset about the loss.
"It kind of is a moral victory to get that under my belt, and I'll start working from here," said Cain. "It was a relief. It was good to get that first start out there to see what these guys can do."
Manager Felipe Alou said Cain will get another shot five days from now at Arizona and then five days after that, and five ... you get the picture. As long as the kid is healthy, he'll keep on throwing on a regular basis.
"He is well equipped," said Alou. "He has all the tools that a pitcher could have, throwing hard, a great presence on the mound, and aggressive. It is a matter of time."