To be sure, it was a lot for the 23-year-old Kelly to digest -- both while it was happening and after the season, at which point he finally took time to process what was a strange season.
"There were some good times and some bad times," Kelly said, smiling.
Kelly is here in camp as one of 10 candidates with a shot at cracking the Padres' starting rotation. Up to three spots are spoken for, occupied by Clayton Richard, Edinson Volquez and Jason Marquis, but there figures to be significant competition for those final spots.
Enter Kelly, who went 2-3 with a 6.21 ERA in six starts for the Padres, starting on Aug. 27, when his contract was purchased from Triple-A Tucson. He started the season with Tucson and made two starts before landing on the disabled list with the elbow strain.
Kelly won his Major League debut against the Braves the day he was recalled, tossing six scoreless innings. A former shortstop who was converted to a pitcher while in the Red Sox organization, Kelly even got a hit that night, all while his father, Pat, a manager in the Reds' farm system, watched from the Petco Park stands.
"I'm not sure there's anything that could trump that moment," the right-hander said.
Kelly, like many young pitchers before him, showed signs of strength and struggles thereafter. All of it is part of a learning curve, according to manager Bud Black.
"You're facing better hitters, facing more talented hitters with better pitch selection, pitch recognition, a better eye; he learned that firsthand," Black said. "Experience is the best teacher for Casey. What we saw was a guy whose stuff plays in the Major Leagues.
"Casey realizes he needs to change speeds and hit his spots. A lot of the things you get away with in the Minor Leagues, you don't get away with in the Major Leagues."
Kelly was initially scheduled to pitch in the Arizona Fall League early this past offseason, though the team decided in the final week of the regular season to shut him down and have him rested and healthy for 2013.
But once the season was over, Kelly wasn't quite ready to close the door on 2012. That's why he and his father spent time this winter watching video from his six starts with the Padres, nitpicking everything, both good and bad.
"The big thing for me is pitch selection," Kelly said. "I watched some film with my dad this offseason and saw different situations where I could have used a better pitch or made a better pitch in situations. And I want to be more comfortable out of the stretch, so I'm working on that."
The Padres understand that Kelly, who is a ground-ball pitcher, will give up his share of hits. His ground-ball rate in those six starts was 55.8 percent, according to Fangraphs. The team would like to see him improve his secondary pitches to go with his sinking fastball.
All told, Kelly was limited to 66 2/3 innings last season between the Minors and Majors. Only once since he was converted to a pitcher before the 2009 season has Kelly eclipsed the 100-inning mark -- 2011, when he threw 142 1/3.
Kelly is ready to tackle a much bigger workload in 2013, and he would like nothing more than to do so as part of the Padres' rotation.
"That's something I don't think is impossible," Kelly said of equaling or besting his innings total from the 2011 season. "I don't think it's that big a jump. I'm good to go and excited about this year."