On Saturday, Rondon entered in the ninth with a 3-0 lead against the Cardinals. Yadier Molina and Jhonny Peralta both singled, but Jon Jay hit a comebacker to start a 1-6-3 double play, then struck out Mark Ellis to end the game.
If the situation was "fluid" before the game, what is it now?
"It's organic still, as they say," Renteria said.
The Cubs have been trying to find the right combination since removing Jose Veras as the closer on April 11 after he blew a two-run lead against the Cardinals. Rondon finished that game for his first save. He notched his second on Friday in a 6-5 win over St. Louis.
Rondon has come a long way. The right-hander was the Cubs' Rule 5 Draft pick in December 2012.
"It feels amazing right now," he said. "Everything I've been working for last year, everything is coming together this year. It's good for me."
The Cubs' other options at closer include Pedro Strop and Justin Grimm. Veras is on the disabled list with a strained oblique and was to begin a rehab assignment soon. In the meantime, Rondon will be ready.
"I need to be more aggressive," Rondon said. "I need to make my first pitch for strikes. That's big for me."
Does he get nervous entering a game with a one-run lead?
"Sometimes in the bullpen, everybody has nerves, but when we go to the mound, everything goes away," he said.
What was the key Saturday?
"I knew I needed to make a good pitch," Rondon said.
"That inning is not for everybody," Cubs catcher Welington Castillo said of the ninth. "[Rondon] made his pitch. It's all him."
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.