"I told him we really needed him in the bullpen," Piniella told reporters in New York before facing the Mets. "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.
"I understand the situation," Zambrano said. "I understand that we need a little, little help in the bullpen. Look, we already lost three or four games in the eighth inning or in relief."
The burly righty said Piniella told him the move was temporary and general manager Jim Hendry was trying to acquire a setup man through a trade. He also was confident he would return to the rotation this year.
"I don't like to be a reliever," Zambrano said. "I don't want to be a reliever but this team needs somebody to step up and help out the bullpen."
The Cubs scouted free-agent pitcher Braden Looper on Monday, a source told ESPNChicago.com.
The Cubs' bullpen ERA entering action Wednesday was 6.15 with a 1-6 record. The bullpen has blown four of seven save opportunities this season.
"You look at these box scores every day around baseball and these games, especially in the National League, they're won in the sixth, seventh, eighth, ninth innings," said Piniella, in his fourth season as the Cubs' manager. "There are few blowouts early. There's some obviously but not [a lot].
"Look, this makes all the sense in the world."
Zambrano, who has started the past six Opening Days, signed a contract extension in 2007 for five years and $91.5 million. He is making $18.875 this season.
"This is not my last year of my contract," said Zambrano, who will be available out of the bullpen beginning Friday. "I just want to get a ring. I just want to help this team and I think this is a situation, I'm here for the team."
The switch to the bullpen gives Piniella a pair of dynamic arms for the back end of games, along with closer Carlos Marmol.
"With Zambrano and Marmol, that's some firepower," the manager said.
"I'm shocked," White Sox catcher A.J. Pierzynski said on "The Afternoon Saloon" on ESPN 1000, when told of the move. "With a guy with his stuff, you can't believe that would happen.
"The way he has thrown the ball over his career, and all that he has done for that organization, to see that happen. ... I've seen that he's thrown the ball not up to what he normally does. He's going to get it turned around. He has a track record. It just surprises me that they would make that move."
Pierzynski understands the politics involved with such a move.
"It's a tough situation," Pierzynski said. "He's not going to want to come out and say anything bad about the organization, and the organization is not going to want to come out and say anything bad about him.
"They still have a long time together with the contract he signed a couple years ago, so it's a tough situation for both. Carlos is a good guy and you wish him nothing but the best."
Information from ESPNChicago.com's Bruce Levine and The Associated Press contributed to this report.