This was something new for the muscular, robust Cain.
"I've had minor stuff -- nothing crazy," he said. "I've had little strains and pulled my groin in 2010, and that's probably the worst thing I've had since I've been playing."
But now the injuries were piling up on him, sabotaging his biggest chance on the Major League stage.
After missing 78 games, Cain finally got back in the Kansas City lineup on July 13, the day after the All-Star break. He had three hits and scored three runs in a 14-inning, 9-8 loss to the White Sox and did pretty well over the next two months. His average for 56 games was .275 with seven homers, 30 RBIs and 10 steals in 10 tries, and he patrolled center field with maximum efficiency.
"It's definitely been tough. Most of these pitchers this was the first time I faced them. So it definitely hasn't been easy, but I feel like I've held my own," he said at the time.
But his medical mishaps were not over. On Sept. 13 came another leg injury -- this time on the right side, a strained hamstring as he dove after Alexi Casilla's double in the 10th inning of a 4-3 loss at Minnesota. His season was over.
Three leg injuries in one season have Cain concentrating on preparing his body to avoid similar shutdowns next year. After an October wedding to Jenny on a beach in Florida, they've settled in Norman, Okla., for the winter, and he's doing some commuting to Kansas City.
"I'm going to drive up here once a month and stay here for a few days and just try doing stuff to strengthen my legs to prevent some of these injuries," Cain said. "That's my main focus, just to come here and work on certain areas that I'm weak in, and continue to do it throughout the season and stay healthy."
Cain made his first trip to KC in November, and he's also working out daily in Norman.
"I've been running and sprinting, and my legs are actually feeling really good right now," he said.
Cain was the heir apparent to center field when the Royals traded Melky Cabrera to the Giants last winter. When Spring Training starts anew next February at Surprise, Ariz., some competition looms.
All eyes will be on Wil Myers, the top-rated Royals prospect who is believed to be about ready for the Major Leagues. Myers is a right-handed power hitter who, for Northwest Arkansas and Triple-A Omaha combined, pounded 37 home runs with 109 RBIs and a .314 average.
Then there's fleet Jarrod Dyson and September callup David Lough, who could contend. There's also the possibility that Cain could be in the right-field picture with veteran Jeff Francoeur, who had a slow year.
So he'll have a fight on his hands.
"If I do, I do. There's always competition," Cain said. "I never look at it as the job is solely mine, regardless of what people might say. You've got to keep fighting, so I'm going to go about my business the same way I have all my career. Whatever happens, happens."
Cain was part of the famous Zack Greinke trade with Milwaukee which, all things considered, certainly seems to be much more successful for Kansas City over the long term. After all, Greinke is long gone from the Brewers along with infielder Yuniesky Betancourt, while three of the four players sent to KC are still around. Alcides Escobar is the Royals' prized shortstop, and Jake Odorizzi is a rising pitching prospect. Only reliever Jeremy Jeffress has departed, recently dealt to Toronto.
For Cain, 2013 has to be better than his 2012.
"Going to Spring Training I was definitely fired up about winning the center-field job. I had a great spring, and the season didn't start the way I wanted to, and then I got injured and pretty much missed more than half the season," he said. "But next year I'm going to do everything I can possibly do to be ready to go, health-wise, swinging the bat, defensively -- anything I can do. I'll give it all I have."