"I don't know why I'm so optimistic about Zambrano this year," Guillen said. "It's not because he is with us. I said it in December. He has to prove to a lot of people how good he can be."
If he can regain the edge he had in his prime, Zambrano will be a low-risk, high-reward pickup.
In Chicago, the 30-year-old set a Cubs record by being the Opening Day starter for six straight years.
Although Zambrano has had a couple of down years, his overall record is an impressive 125-81 with a 3.60 ERA.
For Zambrano, the timing appears to be right to have a bounce-back season.
"If he continues to get better, I think he's going to be one of the keys for us," Guillen said. "I think he's hungry. I think he's going to show people how good he can be. On top of that, he's going to be a free agent next year. We've got everything for him right now. We're going to let him pitch, and have fun, and hopefully have a good year."
Another reason for optimism is his velocity is on the rebound.
"I gained a few miles per hour," Zambrano said. "Last year, I was throwing 91-92 [mph]. In winter ball, I was throwing the ball 94-95, which is good."
The increase, he says, is partly due to working with a personal trainer, something he didn't consistently do in the past.
"I've been working out with my personal trainer," Zambrano said. "It's helped me out a lot. I feel looser."
While with the Cubs, Zambrano had trainers work with him. But in the offseason, when there was doubt that he would return to Chicago, he decided to go with his own training.
"This time, I was by myself," he said. "Plus, I need to do that because I'm not 20 any more. I need somebody to push me and help work with me."
Thus far in Spring Training, Zambrano has had some encouraging throwing sessions.
On Tuesday, he faced a hitting group of Jose Reyes, Hanley Ramirez and Emilio Bonifacio. The right-hander executed some quality pitches. The only hiccup was mild stiffness in his neck. It wasn't to a point where it caused him to shut down his session early.
Also working in Zambrano's favor is the fact he isn't being asked to be Miami's ace or even No. 2 starter. Josh Johnson and Mark Buehrle, respectively, are locking down those two spots.
Anibal Sanchez, Ricky Nolasco and Zambrano round out the rotation, but their order has yet to be determined.
Zambrano is trying to downplay expectations, and instead is focused on getting his career back on track.
"I'm looking to rebound and help this team," he said. "I had my chance in Chicago. It was great for six years, being the No. 1 pitcher. Here, I'm just looking to help this team and pitch well this year."
If their starting pitching holds up, the Marlins believe they have the pieces to reach the playoffs.
"I think it's too early to tell," Zambrano said. "On paper, we have a good team. If we play the game right and play smart. If we do that, we have everything to win."
Zambrano understands that if he does his part, logs his share of innings and gets positive results, his contract situation will take care of itself.
"I look at it like just another year," he said. "If I go out and enjoy this time, and enjoy this ballclub, and help this club, any other contract will come by itself. I don't have to worry about free agency. I just have to worry about pitching good, and do good with this team, and lead this team to a championship."