Rizzo and Minor League pitcher Zach Cates head to the Cubs for Cashner and Minor League outfielder Kyung-Min Na.
The Padres are hopeful that Cashner, who began last season as the Cubs' fifth starter but missed most of the season with a rotator cuff issue, can be a late-inning reliever in 2012, with the potential of moving into the rotation the following season.
There's a chance that Cashner could slide into the eighth-inning setup role for new closer Huston Street, who was obtained from the Rockies last month.
"We're not going to anoint him a role. We're going to use him as a reliever in 2012 and see where that takes us," said Padres general manager Josh Byrnes.
"He's 25 years old and throws 100 mph on a good day. Hopefully he can be a very healthy and effective pitcher for us."
In Chicago, the 22-year-old Rizzo will be reunited with the general manager who traded for him with the Padres (Jed Hoyer) as well as the guy who drafted him with the Red Sox, Jason McLeod, who is now the Cubs' senior vice president of scouting and player development.
Rizzo, one of four players the Padres obtained from the Red Sox exactly 13 months ago for first baseman Adrian Gonzalez, struggled at the Major League level last season, hitting .141 in 128 at-bats. He did hit .331 with 26 home runs and 101 RBIs for Triple-A Tucson.
Rizzo was essentially made expendable to some degree last month, when the Padres made a deal with the Reds that sent pitcher Mat Latos to Cincinnati for four players, including Yonder Alonso, who figures to get the first crack at winning the starting first-base job.
"The difference between the [Pacific Coast League] and the NL West is pretty vast," Byrnes said. "I tend to think he'll figure it out and become a very good player."
Even with the addition of Alonso and with the potential that teams could try to undercut the Padres in a deal, Byrnes said there was a good market for Rizzo.
"There was plenty of interest," Byrnes said. "Some teams were easier to match up with, young player for young player. This made the most sense. Cashner has immediate value ... he can help our club now and has long-term value."
Rizzo said Friday that he was "excited" to hear about the trade and was looking forward to a reunion with Hoyer, McLeod and Theo Epstein, the Cubs' president of baseball operations, who was the Red Sox's general manager when Rizzo was dealt to San Diego.
"I got called up to the big leagues last year and struggled a little bit. I wouldn't say some people wrote me off, but some people I guess lost some faith in me," Rizzo said.
"For them to still have that faith, with everything they helped me through. It just shows me how loyal they are and how honored I am to play for them."
Cashner, 25, was the 19th overall selection in the 2008 First-Year Player Draft out of Texas Christian University.
Cashner moved fast through the Cubs' system, making his Major League debut in 2010 as he went 2-6 with a 4.80 ERA in 53 games, all in relief.
Cashner made his first start of 2011 with the Cubs on April 5 against Arizona, but he soon suffered pain in his right shoulder. He landed on the disabled list on April 8 with a slight strain of the rotator cuff.
While the injury did not require surgery, Cashner did experience a setback in May when he felt tightness in the shoulder. He then had another MRI, which revealed that he had aggravated the shoulder.
Cashner rejoined the Cubs in early September, and finished the season with 5 1/3 innings in relief -- for a total of 10 2/3 innings for the season. He was pitching roughly every third day in September.
He added 8 2/3 innings in the Arizona Fall League, posting a 12.46 ERA in nine appearances.
Cashner told MLB.com in October that he was looking forward to putting rehabilitation behind him and getting regular work in 2012.
"Rehab is not fun; it's not supposed to be fun," Cashner said. "I've moved past that, and I'm healthy now."
Na, who turned 20 last month, hit a combined .268 over four Minor League stops. Na did not have a home run, knocked in 22 runs and had a .358 on-base percentage in his second season in the Cubs' organization.
"He's a very good athlete," Brynes said. "He can really run and throw and field. We feel like he needs to get stronger and get better with the bat."
Na, who signed with the Cubs as a non-drafted free agent in 2009 for $725,000, has what is considered very good speed and stole 20 bases in 2011.
In addition to moving Rizzo in the deal, the Padres traded Cates, who was selected in the third round of the 2010 Draft out of Northeast Texas Community College. Cates had a 4-10 record with a 4.73 ERA in 25 starts last season for Class A Fort Wayne.