Zambrano, who is expected to pitch next week in the Venezuelan Winter League, has some work to do to get back in the Cubs' good graces.
"From a talent standpoint, I am optimistic," Epstein said Wednesday at the Winter Meetings when told of Guillen's projection. "He's thrown real well in Venezuela, and his velocity is back. As far as his attitude, I think it's in the right place right now."
Epstein met with Zambrano in November, and the message at that time was that the right-hander had to earn his way back. Zambrano exited the ballpark early after giving up five homers Aug. 12 to the Braves, and has not pitched for the Cubs since.
"I listened when I first got here, and a lot of people told me he's been in this place before and it's fallen apart when there's been any kind of adversity on the field or in the clubhouse," Epstein said. "He has to prove himself. It's not enough to say things are better. I told him that to his face when we met.
"I said, 'You have to go out and prove it. Words don't matter any more, it's a matter of actions. There's going to be a lot of people who don't believe it, no matter what you say or do, and it'll be a bigger burden than usual on you until your last strike.'"
It's not just important to the Cubs that Zambrano is a successful pitcher.
"We laid out the things he needs to do when he comes back, and that includes winning the trust of his teammates, and the rest of it I'll leave between us," Epstein said. "Am I hopeful or optimistic? I think it can work. He's got to demonstrate through his actions and consistent actions that it will work, and he has a chance to be a really good pitcher."
Zambrano has not pitched in Venezuela since Nov. 18, when he took a line drive off his face, which needed 16 stitches to repair. That was his third start for Caribes.
Guillen and Zambrano were involved in commercials in Venezuela, and he said they talk often. They don't discuss the possibility of the right-hander coming to the Marlins, although Guillen joked they do that on the side.
"We don't talk, we just text message each other, and I've got a bet with somebody. I will tell you, he will win a lot of games for the team," Guillen said. "Will I hire him? No, because he belongs to the Cubs."
Guillen said he'll see Zambrano in a few days.
"We talk as a friend, we talk about what happened in the past, yes," Guillen said. "We talk about how 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."
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter@CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.