"I wasn't able to get extension like I wanted to on my pitches," Jackson said. "I don't think it's super serious."
Jackson was making his 31st and final start for the Cubs. He trailed, 2-0, going into the third after serving up a two-run homer to Matt Holliday, and with one out walked two batters. Yadier Molina drove both in with a double, and then scored one out later on Pete Kozma's double. Jackson intentionally walked Adron Chambers to face Adam Wainwright, who hit a RBI single.
Cubs athletic trainer PJ Mainville and manager Dale Sveum then went to the mound, and Jackson exited.
The right-hander took his Major League-leading and career-high 18th loss, and finishes the season with a 4.77 ERA.
"There were some good things in there, too," Sveum said of Jackson's season. "A lot of people want to throw one season away in their career. It's over, and it's just time to start from scratch in Spring Training."
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.