After the first two months, things didn't look great for the first baseman, but he was among the best hitters in the American League over the final four months, which earned him the AL Comeback Player of the Year Award presented by Viagra.
"It's unbelievable. I'm speechless," Giambi said. "I'm definitely honored. It's been a lot of hard work and a long road."
"He got out of the gate slow, but he had a burning desire to show he could return to be the player that everybody had learned to love," said Yankees general manager Brian Cashman. "He had to go through so much this year, so I don't think anybody would have predicted this."
Giambi missed 82 games in 2004 with a pituitary tumor and an intestinal parasite, knocking him down from an MVP-caliber player to a .200-hitting part-timer.
His offseason didn't do anything other than complicate matters, as he found himself embroiled in the BALCO controversy, something which followed him well into Spring Training.
But his .271 average, 32 homers and 87 RBIs helped lift the Yankees into the postseason, as Giambi became a force to be reckoned with once again.
"I know there were other things, but just coming back from being sick, knowing where I was last year when I couldn't get out of bed," Giambi said, "to come in, play well and help this team get to the playoffs, it's unbelievable."
"He had some injury problems in the '03 World Series, and last year was a nightmare for him," said manager Joe Torre. "Right now, he's really enjoying himself. He's probably enjoying himself more than anybody else.
"If you go back 12 months and see where he was, he's worked hard," Torre added. "It's not something that dropped on his doorstep. He's been working through it, emotionally and physically."
The first step in Giambi's return came on Feb. 10, when he flew to New York to speak with the media for the first time since his tumultuous offseason.
"I'm so proud of where he came from and where he is now," Cashman said. "That starts with him flying to New York in the winter to address the media the best that he could, and not running or hiding from anything."
But to truly overcome his demons, Giambi would have to perform where it counted -- on the field.
On May 9, Giambi was hitting .195 with three home runs and six RBIs. A move to the Minor Leagues was being discussed among team brass, and the future didn't appear to be very bright for the former AL Most Valuable Player.
"I kind of always knew I had it inside, which is why I never panicked," Giambi said. "I knew it was going to take a lot of hard work, but I played well in Spring Training and I knew I wasn't that far away. My numbers didn't look very good, but I thought with a few adjustments, it would get better. I never lost confidence in myself."
Torre challenged Giambi as the team embarked on a West Coast trip. The manager would put Giambi in the lineup every day for a week, and based on what he saw, he would decide how the slugger would fit into the team's plans.
"There was a motivating factor to come back, because I knew I could become the player that I was before," said Giambi. "Even through the tough times, I tried not to get down on myself, and that was a drive I used to help me."
Giambi responded, hitting safely in all six games in Seattle and Oakland. He went 3-for-4 on May 17, driving in three of New York's six runs in a shutout win over the A's. His average had climbed to .227, and things were looking up.
His season continued to move in the right direction in June, as he hit .310, though his power numbers (one homer, nine RBIs) were still missing. The one homer was an important one, though, as he belted a walkoff blast to defeat the Pirates at Yankee Stadium on June 15.
In July, it all came together for Giambi.
Giambi hit .355 with 14 homers and 24 RBIs, lifting his average to .286 and re-establishing himself as a force to be reckoned with. The 14 homers marked the most in a single month by a Yankees player since Mickey Mantle did so in July 1961. Giambi also became the first player in the Majors in four years to homer 14 times while hitting over .350 in a month.
"He's punished the ball; I know I'm a little surprised, knowing everything he's gone through and how drained he looked last year when he went home," Torre said. "The stuff he had to deal with in spring, trying to have him not think too much about hitting home runs. All of a sudden, it gradually got to the point where his bat speed was terrific."
Over the final two months of the season, Giambi remained a threat, hitting 13 homers and driving in 41 runs for New York. His .440 on-base percentage and 108 walks led the American League, as pitchers shied away from throwing him anything near the plate.
When the season ended, Giambi's final numbers read .271-32-87 -- not quite the numbers he posted when he won the MVP, but a far cry from the .208-12-40 he posted in 2004.
Last winter, rumors flew around baseball that the Yankees were trying to void the remaining four years on Giambi's contract. Now, with three years remaining on that deal, the Yankees believe Giambi's future is brighter than ever.
"We were in a situation where a major asset of ours had become devalued," Cashman said. "He's put value back in that deal and has returned to being an asset instead of an underperforming one. It's huge. We're getting what we paid for, and he's helping us in the bottom line, which is the win column."
"It's definitely been the most challenging season, no doubt about it," Giambi said. "Even more than being a rookie, when you don't know what to expect. I didn't know what to expect this year after being sick and everything, because New York is a tough town to come back in. To do this in New York makes it even more special."
Mark Feinsand is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.