"I told him we really needed him in the bullpen," Piniella told reporters in New York before facing the Mets, according to the Chicago Sun-Times. "We felt he could do a really nice job for us there.
"He said he would do what's best for the team. I'm very appreciative. We've talked about maturing. This proves it to me."
Zambrano is 1-2 with a 7.45 ERA in four starts, and the move makes room for Ted Lilly, who is returning to the Cubs' starting rotation on Saturday in Milwaukee after undergoing offseason shoulder surgery.
"[Piniella] told me that probably [assistant general manager] Randy [Bush] or [GM] Jim Hendry was looking for an eighth-inning guy," Zambrano told the Tribune. "And when they find a guy and trade [for a] guy and get that guy here, I will be back to the rotation.
"Right now they need somebody to step up in the eighth inning. I don't like to be a reliever. I don't want to be a reliever, but this team needs somebody to step up and help the bullpen."
The Cubs scouted free-agent pitcher Braden Looper on Monday, a source told ESPNChicago.com.