Piniella has talked to pitching coach Larry Rothschild and bullpen coach Lester Strode, but hasn't found an answer.
Marmol had a good outing Wednesday against Atlanta, but the day before, he walked two and gave up a run in the Cubs' 6-5 loss to the Braves.
"There's nothing wrong," Marmol said.
Piniella said he has no plans to switch roles and use Angel Guzman as the eighth-inning pitcher.
"Right now, Guzman's really pitching well and let's leave that alone," Piniella said of the right-hander, who has thrown 11 1/3 scoreless innings dating back to May 8. "We need to get Carlos straightened out there in the eighth inning and we need to make the ninth inning not as exciting."
The Reds had two on and two outs in the ninth against Kevin Gregg, getting an extra out when Jerry Hairston Jr. was safe at first on a wild pitch on strike three. Gregg got Chris Dickerson to pop up and end the game.
"My mouth is cotton dry," Piniella said. "I feel like a water moccasin."
The Cubs had chances, stranding nine in the game, and nearly blew it as they walked seven. The rule of thumb, Piniella said, is if you have a baserunner and a half per inning, you usually lose.
"You're fortunate to win a 2-1 ballgame walking seven people," Piniella said.
The Cubs made some solid defensive plays to escape jams, including a double play in the sixth in which first baseman Derrek Lee went into the stands to catch a pop up and Zambrano was quick enough to cover first and get Dickerson.
Marmol is another matter. What to do? Piniella planned on talking to the pitching staff -- and the pitcher -- again.
"I'm going to keep fighting," Marmol said. "I believe in my stuff. I trust my stuff."