Weakened by the disease that now bears his name, Lou Gehrig stepped onto the field July 4, 1939, at Yankee Stadium and told the world, “Today, I consider myself the luckiest man on the face of the Earth.”

The problem: Most of the world didn’t hear it.

That changed when actor Gary Cooper, with a healthy dose of artistic liberty, stepped into the Iron Horse’s shoes in “The Pride of the Yankees.” The 1942 film, released one year after his death at age 37, immortalized Gehrig.

No. 4’s story and legacy may never be duplicated in the digital age, author Richard Sandomir argues. His book, “The Pride of the Yankees: Lou Gehrig, Gary Cooper, and the Making of a Classic,” is set to hit stores Tuesday.

The mythology surrounding Gehrig could not have existed if the Hall of Fame first baseman had played today, Sandomir argues, because social media and 24-hour news cycles provide more information on modern players.

“Everybody’s revealed all the time,” Sandomir said. “By the end of the season you know plenty about a great rookie, and by the time he retires you’ll know far too much.”

Only snippets of Gehrig’s original speech exist, and written accounts of it vary. It’s unlikely a complete version of that speech, if it happened today, would be missing and open the door for Hollywood to take dramatic license.

“Because the [full] original speech has not been preserved ... the movie fills in all the blanks,” Sandomir said. “It gives you the full speech as recreated by the screenwriters and by Cooper.”

An actor at the peak of his craft, Cooper adroitly portrayed Gehrig as a humble, courageous man.

“This was a guy who played dignity,” Sandomir said. “He played this role. You couldn’t find a better person.”

Sandomir spent considerable time digging through letters and alternate scripts. His book covers numerous topics, such as Cooper’s limited baseball abilities and Gehrig’s relationship with mother Christina and wife Eleanor.

“What ‘Pride of the Yankees’ did initially was to kind of say to the world a year after [Gehrig] was dead, ‘We’re not forgetting him,’ and 75 years later we still have this,” Sandomir said.

That’s not to say other athletes can’t be immortalized. But times have certainly changed.

“Players are not shielded anymore as much as they try to be,” Sandomir said. “Gehrig was the perfect person to die tragically and to have his image be perpetuated as positively and brilliantly as it was.”