"I feel 100 percent and have my velocity," Rondon said Sunday. "Now I feel like everything is coming back."
The right-hander has pitched two innings in three of his 11 games, including Saturday against the Nationals.
"He's been bouncing back," Cubs manager Dale Sveum said. "It kind of all worked out real well, because our starters did such a great job in the first month that we were able to ease him into that role that he hadn't been able to do the last few years."
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.