Cubs and Marlins officials have yet to publicly comment on the reports.
If the Cubs do move Zambrano, it would end a tumultuous career in Chicago for the right-hander, good enough to throw a no-hitter in 2008, but also frustrating because of his antics.
Zambrano, 30, would have to waive his no-trade clause for such a deal to be completed. The volatile right-hander is coming off a 9-7 season that ended abruptly Aug. 12. He served up five home runs to the Braves that day, then was ejected for throwing inside to Chipper Jones and left Turner Field early. Zambrano told teammates he was retiring, but later recanted.
He was subsequently placed on the disqualified list and did not pitch again for the Cubs. This offseason, Zambrano made five starts for Caribes in the Venezuelan Winter League and last pitched Dec. 27, giving up one run over 4 1/3 innings.
Zambrano traveled to Chicago in November to meet face to face with Theo Epstein, who took over as Cubs president of baseball operations in late October. At that time, Epstein gave Zambrano steps to follow to get back in the team's good graces.
"I told him we'd give him the right to earn his way back to being a Cub," Epstein said in November after the meeting at a Wrigleyville restaurant. "Nothing would be given to him but he could earn his way back from very hard work this winter, through rebuilding relationships man to man with all of his teammates, and through some other steps that we discussed.
"We're not welcoming him back unconditionally at all but we're going to give him the right to earn his way back to being a Cub."
Neither Epstein nor Zambrano's agent, Barry Praver, would go into detail as to what those steps were.
On Wednesday during a fan question-and-answer session, Epstein talked about how Zambrano has to change to fit into what the Cubs are trying to do.
"The Carlos Zambrano of 2011 and years previous can't fit into the culture that we have here," Epstein said. "Change needs to happen and change will happen. Either he'll change and buy in and fit into this culture -- and I understand there are a lot of skeptics around about that, and I understand that, and frankly, I'm skeptical as well.
"He needs to prove to us that he can change and be part of this culture," Epstein said, "or we'll change the personnel and move forward with people who are proud to be Cubs and treat their teammates with respect, treat the fans with respect and can be part of a winning culture in the Cubs' clubhouse."
In order to move Zambrano, the Cubs knew they would most likely have to absorb a large portion of his contract. The right-hander's deal includes an option for 2013 that could vest depending on his health and performance, and the Marlins will be responsible for that.
If the deal is made, new Marlins manager Guillen would have a projected rotation of Josh Johnson, Mark Buehrle, Anibal Sanchez, Ricky Nolasco and Zambrano.
During the Winter Meetings in Dallas, Guillen predicted Zambrano would win 14 to 16 games. The two talk often, Guillen said, but he stressed at that time that they did not discuss the possibility of the right-hander coming to the Marlins.
"We talk as friends, we talk about what happened in the past, yes," Guillen said. "We talk about how much better it's going to be, yes. We talk about what kind of pitcher he can be, yes. But talking about the Marlins, I never did."
Zambrano has a career 125-81 record and 3.61 ERA in 319 games. His 125 wins with the Cubs rank 15th all-time, and his 1,542 strikeouts are second in team history. He would help the Marlins at the plate as well. Zambrano has 23 career homers, the most by any pitcher in Cubs history, and the most since Cardinals Hall of Famer Bob Gibson retired in 1975 with 24.
Volstad, 25, was 5-13 with a 4.89 ERA in 29 starts last season with the Marlins, striking out 117 over 165 2/3 innings. In 103 games over four seasons in Florida, he was 32-39 with a 4.59 ERA. Volstad earned $445,000 last season; he will not be a free agent until after the 2014 season.