The Marlins officially signed Furcal to a one-year, $3 million contract on Friday.
"He's working out. He's doing the rehab," Jennings said. "He can throw now. All that has checked out. He passed our medical. All is well.
"He's not actually doing full baseball workouts. He's doing strength and conditioning workouts and starting to throw to get his arm stretched out."
The two parties enter 2014 in similar situations -- both are looking for bounce-back seasons.
Furcal, who has been to the playoffs in 12 of his 14 seasons, was looking for a fresh opportunity. And the Marlins, who lost 100 games, are striving for dramatic improvement.
Furcal missed all of 2013 due to Tommy John surgery on his right elbow. With the Cardinals in 2011, he was the regular shortstop on a World Series title team. In 2012, he appeared in 121 games for St. Louis, batting .264 with a .325 on-base percentage.
Now with the Marlins, he will take on a leadership role and be the regular at second base, a position he has only played in 36 games in his career. He last saw action at second in 2004.
If necessary, he informed the Marlins that he could give shortstop Adeiny Hechavarria a day off every so often and play short.
"He even brought that up," Jennings said. "He said, 'If Hech needs a day, I can slide over and still do it.'
"He loves Hechavarria. He talked openly about Hechavarria. He had so much positive stuff. I can't say enough about his energy, leadership, his character, the whole thing."
Jennings added that Furcal may no longer have upper-tier arm strength, but it still should be better than average.
Furcal informed the Marlins that he should be ready to go when Spring Training gets underway in mid-February.
"Everything checked out medically," Jennings said. "We met with him face to face. He lit up the whole room. He was unbelievable, the energy, the things he said."
Furcal has been around winning his entire career.
On Monday, the Hall of Fame inducted three managers -- Bobby Cox, Tony La Russa and Joe Torre. Those three happen to be the only managers Furcal ever had before joining manager Mike Redmond's team.
"After bringing him in and meeting face to face with him, he's extremely hungry," Jennings said. "After not being part of last year, and those guys [Cardinals] going to the World Series, he brings great energy."