"It's something that happens in baseball. It can happen to anyone," Kelly said of the trade. "When I got the news, I was shocked and surprised. I found out it was Boston and knew I was going from one great baseball town to another.
"These guys traded for me and I am looking forward to this opportunity to help this team win. I want to go out and pitch my butt off every time I am on the mound and compete and win."
Ironically, Kelly will make his Red Sox debut against the Cardinals on Wednesday at Busch Stadium.
"It is going to be different. It will be weird," Kelly said. "I have to go out and pitch like I don't know those guys."
Kelly is now one of the most experienced pitchers in a Red Sox rotation that is full of young, raw talent.
Many Red Sox fans will remember Kelly as the Cardinals' Game 3 starter in the 2013 World Series. He tossed 5 1/3 innings, allowing two runs on two hits in a no-decision.
"My motivation is to beat the other team any time I pitch. You don't want any of those outside factors creeping in," Kelly said. "I learned that pitching against the Red Sox in the World Series. Media, fans, other people, I just tried to go out there and have fun."
Kelly has made just seven starts this season, after spending three months on the disabled list with a left hamstring strain. Since his return in July, the right-hander is 1-1 with a 7.32 ERA.
"I was on the DL for almost three months. It's been a month now. I have to keep competing. I like to win as much as I can. I like to sprint after balls. Though I am in the American League now, that's what I did in the National League," Kelly said. "I play the game at 100 percent. People don't think pitchers are very athletic, but that is my style. Any way I can help my team, I will do it."
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, and follow him on Twitter @IanMBrowne. Quinn Roberts is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.