There is a picture of Lou Gehrig taken on May 2, 1939, the day he took himself out of the lineup “for the good of the team.” He is leaning on the dugout steps, gazing out at the field on which he would never again play. He looks wistful, almost serene, an extraordinary accomplishment given that he had been crying just moments before.

By June, a repeat of that famous photograph featuring Jason Giambi seemed like something that might come over the wire at any moment. The burly first baseman and designated hitter had batted .234/.379/.355 through May 31. The on-base percentage was, as always, excellent, but this was actually a bad sign rather than a good one. That’s because the old, potent Giambi could hit for both average and power, and was extremely selective. The new Giambi had been reduced to the equivalent of a tennis line judge, unable to do anything but watch balls go by and decide whether they were in or out.

As a best-case scenario, he might turn his shoulder and get hit by a pitch. The new Giambi’s passivity caused one commentator to label him the Yankees’ “designated looker.”

Reaching base is the essence of baseball offense and so should not be taken lightly, but Giambi’s impotence at a power position was damaging to the Yankees. Worse, once on base, Giambi moved like he was carrying an elephant on his back. He looked old, tired, finished.

When it became obvious to Gehrig that the Yankees were keeping him in the lineup out of respect, he quit. By June, it seemed obvious to everyone but Giambi that the Yankees were keeping him in the lineup out of a different kind of respect – respect for the expensive contract that will keep Giambi in George Steinbrenner’s pay until 2008. Giambi showed no signs of following in the Iron Horse’s footsteps.

Since the beginning of 2004, a tortured, controversial season during which the big man spent more than two months on the disabled list with an officially unrevealed, possibly steroid-related illness (among other problems), Giambi had hit roughly .215/.345/.369.As folksinger Phil Ochs might have put it, these are rehearsals for retirement, particularly for a defensively challenged first baseman. Though Giambi is only 34, he had been so lost for so long that it was quite reasonable to assume there would be no comeback. Players whose skills desert them to the level that Giambi’s had almost never find them again.

It is all the more amazing that in June, Giambi rediscovered his ability to hit for average, batting .310/.431/.474.Even then, he hit just one homer, so the complete package still hadn’t been recovered. In July, he has found it and more, exceeding his peak numbers by hitting .379/.507/1.034 with 11 home runs. Giambi went from millstone to Bonds country in the space of three weeks.

Some players who suffer partial outages seem to come back. At age 25, Twins third baseman Gary Gaetti, who had hit 25 and 21 round-trippers in his first two seasons (more impressive then than it is now) forgot how to hit home runs. In 1984, he played in 162 games and hit only five. The next year, he rebounded to 20. Overall, he hit 307 post-power-failure home runs. Wade Boggs batted an unthinkable .259 as a 34-year-old, then rediscovered his stroke and hit .307 over the final seven years of his career.

More often, though, once the skills are gone, they’re gone. Take Dale Murphy, the sole star of the dreadful 1980s Atlanta Braves. At age 31, he was the owner of .279/.362/.500 career averages (again, better then than now), 310 career home runs, five Gold Gloves, and two MVP awards. He was going to the Hall of Fame – or at least it seemed. From that point on, Murphy pancaked, batting just .234/.307/.396 over his last 661 games. He was given six years to find his stroke and failed.

The same goes for most players who – as Giambi did in 2004, for all intents and purposes – have taken a year off. Boston Red Sox MVP Jackie Jensen was a career .279/.369/.460 hitter prior to taking a year off to avoid plane travel. When he came back, he batted .263/.350/.392 and was gone for good. Dave Winfield was a career .287/.357/.481 hitter when a herniated disc robbed him of the 1989 season. When he came back, Winfield hit .267/.337/.453 – still good, especially for an athlete in his late 30s, but not what he had once been, especially since the seven-time Gold Glove winner had completely lost his mobility.

It would be foolish to make the same mistake twice as far as forecasting Giambi’s trends. It would be wrong to conclude that he was back because of an extended hot streak; wrong for the same reasons that a very extended cold streak had suggested he was gone. Yet at this stage, the Yankees probably have to take his transformation at face value. The rationalization works this way: Unlike, say, Dale Murphy, Giambi’s skills hadn’t disappeared, or atrophied, but were simply misplaced. The very fact of what Giambi has achieved in the last seven weeks is its own proof; if he wasn’t still capable of doing it, it wouldn’t have happened.

Giambi will probably still be a burden at the end of his long contract, when he’ll be so slow his uniform number will be a truncated triangle. That’s for the future. For now, the next year or two look rosy, and the balance of the season is a great deal simpler. Rather than needing to find two power bats (because first base or DH, whichever Giambi is not playing is still a problem), they need one or none – center field and relief pitching become the priority.

Giambi’s situation is without precedent. Credit goes to batting coach Don Mattingly – whose own skills were destroyed by injury at age 28, never to return – for patiently working with the former MVP slugger, and Giambi himself for not giving up.

Greater credit still should go to dumb luck; when F. Scott Fitzgerald said there are no second acts in American lives, he must have been talking about 30-something ballplayers. Giambi is the sole exception.

Mr. Goldman is the author of “Forging Genius,” a biography of Casey Stengel, released this year.