Kelly, widely regarded as the Red Sox's top prospect, had been weighing that question in his mind since he was drafted in the 2008 First-Year Player Draft. When it finally came down to picking one over the other, Kelly essentially asked himself this: "How will I get to the Major Leagues the fastest and how will I stay there the longest?"
It all became so clear at that point, with Kelly telling the Red Sox earlier this week that he was trading in his bat for a full-fledged push to the Major Leagues, one that could well land him a spot in Double-A Portland's starting rotation this season.
"It's such a unique situation that I've been in for the last five years by doing both positions and everything," Kelly said in a conference call with reporters on Wednesday. "The biggest thing for me was, I just sat down and asked myself -- put the emotion aside -- and asked myself what my main goal was with baseball, and it was to make it to the big leagues. Pitching just seemed like it was going to be faster and a longer career in the big leagues for me. Once I asked myself that, it was a pretty easy decision."
Though he is 20 years old, Kelly has seemed more advanced all along from both a pitching and maturity standpoint.
Red Sox general manager Theo Epstein and director of player development Mike Hazen met with Kelly last week to discuss their view on where he was as both a pitcher and a shortstop. Kelly gathered that information, digested it and then came to his own decision.
"When you're dealing with situations like this, as Casey said earlier, it was very unique," Hazen said. "I think when we first embarked on this situation, we knew it would be very, very challenging, and if it weren't for Casey's maturity, I don't think it works the way it does. Because Casey, I think, had demonstrated that maturity, we felt very comfortable wanting Casey involved in the process as much as possible. And, I think, at the end of the day, he is very talented, he's very athletic, he is skilled in both areas, and this wasn't an easy decision for anyone as we put all the pieces together and kind of weighed all the factors for both sides."
Kelly admitted that making a firm decision has taken a burden off him.
"It's been a huge burden -- I think ever since the pitching was done last year and me going to play shortstop," said Kelly. "Since I was so successful as a pitcher, everyone couldn't figure out why I wanted to play shortstop. I think after making this decision, it's like a weight has been lifted off my shoulders and now I can just focus on one position instead of answering questions about what position I'm going to play. It's going to be on how I perform. I think it's going to be a little easier to focus on one position and one thing."
Kelly is thrilled to know he will have the chance to take part in Major League Spring Training two months from now.
"I'm really excited just to be able to be there and to join that team is going to be unbelievable," Kelly said. "Just watching those guys and how they go about their business and how they handle things is going to help me so much in my development as a pitcher. And then I get to sometimes play, which is pretty cool. I'm really excited about the opportunity to get to big league camp."
What does Kelly think he needs to do to establish permanent residence in the big leagues?
"I think just throw innings," said Kelly. "Last year, I threw 100 innings. I threw half the year. Each time I get on the mound, [I'm] trying to learn something and take something from my last outing. I think just getting my pitches to develop more -- my curveball, my changeup -- just keep working on those things on a day-to-day basis and fully focus on that is going to help me even more."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.