"I don't think I've stopped smiling since I got off the mound," said Kelly, widely regarded as the best pitching prospect in the Red Sox's farm system. "It was a good first outing. To have the crowd and some of the people behind me playing defense behind me was a tremendous honor."
Just like Jonathan Papelbon once had his first Spring Training inning with the Red Sox back when he was a much-hyped Minor Leaguer -- and so did Jon Lester, Clay Buchholz and Daniel Bard -- Wednesday afternoon against Northeastern University was Kelly's first taste of pitching with a Major League team at his back.
Of those 10 pitches in the 1-2-3 first inning, seven were strikes. Two of the outs were strikeouts, both on changeups.
"I was very, very nervous going in," Kelly said. "Once I got on the mound, kind of the competition takes over and you want to go out there and do your best."
Keep in mind that Kelly's heart hadn't felt this type of beat since the Futures Game in St. Louis last July. That, remember, was the last time he pitched in a game.
Kelly finished the 2009 season playing shortstop, and he did the same in the Arizona Fall League. But in December, he made the decision to focus exclusively on becoming a pitcher, which has put him on a faster track to the Major Leagues.
It was news to Red Sox catcher Victor Martinez that Kelly, who is the 28th-ranked prospect according to MLB.com, was once conflicted as to what positional path he wanted to pursue. The way Martinez looked at it, that was a full-fledged pitcher he was calling signs for against Northeastern.
"I just heard that. That's amazing," said Martinez. "Shortstop [and then] going to pitch? That's amazing. I thought he signed as a pitcher. His delivery and all that, it was pretty good."
Not only was Wednesday the first time Kelly pitched for the Red Sox, but it was the first time he pitched against a college team. Though Boston drafted Kelly in the first round of the 2008 First-Year Player Draft, he also had the option of accepting a full scholarship to play football at Tennessee.
Kelly might not have even thought much about how he was facing a college team for the first time if not for a telephone conversation he had with his father, Pat, a former Major League player and current coach in the Cincinnati Reds' farm system.
"He called me last night and gave me some grief, saying it was my college debut," said Casey Kelly. "We joked around a little bit about that. He just told me to go out there and have fun. It's just like the game I've been playing my whole life."
And despite the eight months that passed since the scoreless inning he threw in St. Louis, Kelly, who might start this season at Double-A Portland, immediately felt back at home on Wednesday.
"No, it actually felt like I had been doing it for a while now,' said Kelly. "It felt like I really didn't take an eight-month break. It felt good out there. It's kind of like riding a bike. Once you do it once, it kind of comes back to you pretty fast."
There is a buzz that surrounds Kelly, making it easy to forget he is all of 20 years old.
"You get up this morning and I saw [Tennessee football coach] Phil Fulmer getting interviewed about him," said Red Sox manager Terry Francona. "People are caring about what he's doing. I walked by him yesterday and somebody had mentioned something about our signs, and I saw his eyes. There was a moment of, I think, panic. He's getting a lot thrown at him in a hurry. I do think it's amazing how when you look at him, he doesn't look like a kid that just turned 20 that hasn't pitched."
Wednesday is a glimpse of the life he hopes to settle into in a couple of years.
"I was just excited to put that Red Sox uniform on and get to play on the same field as some of the big leaguers -- Bill Hall, Jacoby, Jed Lowrie and Gil Velazquez," said Kelly. "To throw to Victor Martinez, I think I was more nervous about throwing to him than facing hitters. I felt good out there and I'm excited to get that first outing out of the way."
And when Kelly finished his inning, he got congratulatory handshakes and pats on the back from people he's used to watching on television.
"Yeah, everybody was congratulating me," Kelly said. "That was kind of the coolest part. Just having John Lackey or Clay Buchholz come up and say, 'Good job' to you, that was a special moment for me."
Eventually, the Red Sox won't have enough innings to go around and Kelly will be sent to Minor League camp.
But he will soak up his experience as long as it lasts. Up next for Kelly will be a relief stint against the Orioles on Sunday at 1:05 p.m. ET in Sarasota, a game that Buchholz will start and will be aired on MLB.TV.
"He has some great stuff," said Martinez. "He was throwing his fastball in and out, mixing it with his curveball, changeup. He only threw one inning, but he threw pretty good pitches, quality pitches."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.