Drafted by the Red Sox in the first round of last month's First-Year Player Draft, Kelly also has the option of playing quarterback at the University of Tennessee.
"It's a hard decision, but there's no wrong decision," said Kelly. "Either way, it's good for my future and everything like that."
As the 18-year-old stood behind the batting cage and watched such established stars as Manny Ramirez and Mike Lowell take batting practice before Wednesday's Red Sox-Rays game at Tropicana Field, he was simply soaking in the moment.
"Only certain people get to go through what I'm going through," said Kelly, who was wearing a Red Sox cap. "Obviously, I'm having fun with it. I've just been having fun."
While meeting and greeting members of the Red Sox at Tropicana Field, Kelly wasn't far from his home, in Sarasota.
But he was in Boston on Tuesday, and actually got to play at Fenway Park in some sort of intrasquad game that included Draft picks and/or potential signees.
"It was awesome, playing in that stadium," he said. "The Green Monster. Just all the people that have played there, to play on the same field as them was just awesome."
Though Kelly hopes to one day play in front of a much bigger audience at Fenway, he enjoyed his unofficial debut.
"I think I was 2-for-5 with a double and a single," he said. "Made a nice play in the hole. Not too bad. My double was off the Monster, but [I] didn't get it over it."
The visit to Boston was simply an introduction, as negotiations with Kelly haven't really started. The Red Sox have until Aug. 15 to sign him.
"We really haven't talked," he said. "Right now they're trying to get other guys signed and stuff. Probably, in the next couple of weeks, we'll start talking more and more about the negotiations and stuff."
Kelly has also been keeping in touch with his contacts at Tennessee, including head football coach Phillip Fulmer.
"I've been talking to them every other day, and stuff like that, keeping them updated. They know what's going on," said Kelly. "They said, 'It's your decision.' They support me either way, which is awesome."
And if Kelly does decide to start a professional baseball career with the Red Sox, he will then have another decision to make: shortstop or pitcher?
Then again, the Red Sox figure to have a good bit of say in that one.
"I like to play shortstop," Kelly said. "Pitching is something I like to do also. That will be part of the negotiations also. We haven't really talked yet. We'll just have to wait and see."
Which position does Kelly enjoy more?
"Right now, probably shortstop, but it depends on how the future looks and everything," he said.
No matter which path Casey chooses, his future is looking pretty good.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.