Toronto general manager Alex Anthopoulos initiated talks with Marcum last week, but negotiations were still in the feeling-out stage when interest grew in a potential trade with the Brewers.
Marcum told MLB.com's Adam McCalvy the trade came as a surprise, especially because his agents, Rex Gary and Jim Turner, had just begun negotiations with the Blue Jays about an extension.
"It kind of caught me off-guard," Marcum said of the news Sunday that he'd been traded. "It was more of a phone call than a formal offer, just to see if we were interested, and two days later, the trade happened."
For his part, Anthopoulos declined to comment on possible negotiations between the two sides, but he admitted that Marcum was initially taken aback when informed he was being dealt to the Brewers.
"Shaun was someone [that] I think was pretty adamant -- I think he came out publicly -- and said he loved his time in Toronto, wanted to be in Toronto long term and that's part of what kind of teammate he was," Anthopoulos said.
"I think it shook him a little bit, the news of the trade. But after he thought about the Brewers and where they're headed, and the players they had -- Shaun's about winning. He knows he has a tremendous opportunity to win."
Marcum, who will turn 29 on Dec. 14, is entering his second season of arbitration eligibility and remains under club control for the next two seasons.
He was in line for a raise on the $850,000 he made in 2010, but this move was never about the money for the Blue Jays. It was about acquiring a premier offensive prospect to complement Toronto's depth of young arms.
"For us, it's not only about one year," Anthopoulos said. "It's about trying to build a sustainable winner. I think that's truly because of the upside of our market, being across Canada and our ownership.
"If we build it correctly, we should be able to build a consistent winner for the long run. It's going to require taking some chances and gambling a little bit on prospects and players like Brett Lawrie who have high ceilings but haven't done it yet [in the Majors]."