Epstein has agreed to a deal with the Cubs and would leave Boston with a year remaining on his contract, the person said on condition of anonymity because the negotiations are confidential. The deal is reportedly for five years.
Whether the Cubs have to send the Red Sox cash, prospects or both was not clear, though various reports say the two sides were not in agreement Thursday. The talks continued Friday for a second day.
Any deal would have to be approved by Major League Baseball.
Red Sox owner John Henry said Friday he didn't want Epstein to leave.
"I'd love to have Theo back. I would have loved for Theo to have been our general manager for the next 20 years," Henry told WBZ Radio in Boston on Friday.
"That was my hope. That would have been my hope. But you don't always get what you want. I did everything I could, personally — and so did (Red Sox executives) Tom (Werner) and Larry (Lucchino) to make that happen. But the fact that is, and I think people don't understand this, the fact is that being the general manager in Boston or being the manager in Boston is a terrifically tough job."
Epstein got the job at age 28 in 2002.
"He never saw the general manager's role as longer than 10 years for himself," Henry said. "I mean, maybe he did early on, but certainly after a few years he knew the stress of this job was too much."
The potential arrival of Epstein has Cubs fans in a tizzy. It has been greeted with even more fanfare than the hiring of marquee managers Dusty Baker and Lou Piniella in an effort to end a World Series championship drought that reached 103 years after a 71-91 finish this season.
Epstein, now 37, was at the helm when the Red Sox ended an 86-year World Series championship drought in 2004 and won the title again in 2007.
Cubs chairman Tom Ricketts fired GM Jim Hendry in July after another disappointing season and wants a GM schooled in both old-school scouting and the new world of statistical analysis. The Yale-schooled Epstein would fit both requirements.
Another potential stumbling block could be which members of the Red Sox's organization — if any — Epstein would be allowed to take with him to Chicago.
Epstein also has his critics after the Red Sox collapsed in September and missed the playoffs for a second straight year. Some of his big-contract players like John Lackey, Edgar Renteria, Daisuke Matsuzaka didn't measure up while another, outfielder Carl Crawford, struggled in his first season with the Red Sox.
Epstein's first order of business — if and when the deal is complete — could be to determine the future of manager Mike Quade, who has one year left on his two-year deal.
AP Sports Writer Jimmy Golen in Boston contributed.