Zambrano, who has made 11 relief appearances since moving to the bullpen April 22, will throw a simulated game Monday at Wrigley Field. He's expected to throw 60-65 pitches then, and the Cubs won't know if he'll need another simulated start until they see how he feels Tuesday.
However, to allow Zambrano time to prepare for Monday, the Cubs will be a pitcher short in the bullpen.
"When I give my players my word on something, I follow through with it," Piniella said. "I told [Zambrano] that we're going to extend him on Monday. I told him I needed him in Philadelphia and at the beginning of this series. He's done his part, and I'm going to do mine."
Zambrano, who did pitch 1 1/3 innings of relief Friday against the Rangers, has made it clear he wants to be back in the rotation, even though he's done well in his last three relief outings. In four starts, including Opening Day, Big Z was 1-2 with a 7.45 ERA.
"His preference is to start," Piniella said. "I asked him to do something [and pitch in relief]. He tried it; he wasn't as comfortable with it. We'll get him back in the rotation and let him pitch; let him throw the ball the same way he's throwing now in the rotation and he'll help us win some ballgames."
So what's the Cubs rotation after Sunday?
"As of now, I don't know exactly what the rotation is," Piniella said.
Expect Ryan Dempster on Tuesday when the Cubs open a six-game homestand against the Dodgers and Cardinals. That way, he'll stay on his five-game schedule.
Gorzelanny was in Chicago to be with his wife, Lindsey, who gave birth to their first child, son Gavin, on Friday. He threw there and had no problems with his left ring finger, which was hit by a comebacker on Thursday in Philadelphia.
A six-man rotation is not an option. Everything will hinge on how Zambrano feels Tuesday.
"What I told Carlos is to be honest with me," Piniella said. "[Tell me], 'Look, I'm ready,' or 'Look, I need one more [simulated game].' The thing about this is by pitching him Monday, we're pitching with one pitcher less and we don't want to get into that routine for a long period of time."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.