"It was tough," Kelly said recently. "Obviously, you never want to be hurt and be out for three and a half months. Looking back at it now, it doesn't seem like I was there that long. But when you're in it ... every day feels so long."
Kelly need not worry about Arizona now, not unless he returns there with the Padres later this month as part of the starting rotation.
San Diego promoted Kelly on August 27, and he responded by tossing six shutout innings during his Major League debut against the Braves at Petco Park.
"I couldn't imagine anything else," Kelly said afterwards. "I would have been happy with anything, but to come out with the win, it's just unreal."
To be sure, it was a very strong start to what the Padres will hope will be a promising career, although there figures to be bumps in the road.
One of those bumps occurred in Kelly's last start on Sept. 2, when he allowed five earned runs to the Rockies in a loss at Coors Field. Kelly allowed seven runs in the third inning of that game, but he showed his manager and teammates something with the way he pitched and handled himself after that rough inning.
For as bad as that third inning was, Kelly didn't let it consume him. In fact, he retired the next six hitters that he faced before allowing a single to Ramon Hernandez to start the sixth inning.
"That says a lot about Casey," Padres manager Bud Black said. "He was fine. He wasn't rattled. He kept battling. He just went out there and kept pitching."
Black wasn't the only one impressed with the way Kelly bounced back.
"I thought he showed a lot of maturity and resiliency," Padres third baseman Chase Headley said.
As Black has been quick to caution, Kelly is still a young player trying to find his way as a pitcher. He was, after all, drafted as a shortstop by the Red Sox before being converted to a pitcher in 2009. To date, he's made only 73 starts in the Minor Leagues.
The plan is for Kelly -- the key cog in the December 2010 deal that sent Adrian Gonzalez to the Red Sox -- is to pitch every fifth day between now and the end of the season. He's also scheduled to pitch in the Arizona Fall League, though that plan could change.
"I think I'd be going to get more innings, just because I might end up under 100 innings after this year, and I don't want to be behind heading into next year," Kelly said. "We'll see how the rest of the season goes, and then if I need more innings at the end of the year, then I'll go there to pitch."
As long as he's healthy and pitching every five days, Kelly doesn't care where it happens, even if it's -- gulp -- Arizona.
"That was tough, but everything I did there to prepare was worth it," Kelly said. "Obviously, you don't want to be hurt and be out three and a half months, but looking back now, it doesn't seem like it was too long. But when you're in it, every day feels so long."