People are just tossing around the phrase “Roman Legion” as if it meant exactly one static thing, unchanging through the whole history of Rome. But that history lasts about eight hundred years, which is the distance between us and the signing of the Magna Carta. The early Republic had soldiers who fought much more like Greeks, in tight spear wall phalanxes with some heavy armor, and some cavalry and auxiliary slingers and archers to skirmish before the main battle and on the wings. It wasn’t until about 100 B.C. that Gaius Marius reformed the legions into something we’d recognize as classically Roman, men with uniform armor, short swords, throwing spears, and the like, supported by archers, slingers, and other light troops, with some light and heavy cavalry supporting them. Over the next few hundred years, cavalry greatly increased in importance and the legions almost entirely lost their Italian character, as citizenship was granted to any number of Germans, Gauls, Iberians, Greeks, and other non-Italians. By the end of the Western Empire (late 400s AD, by most reckonings) barbarian cavalry, sometimes Roman citizens but just as often non-citizen mercenaries, had started to dominate the field, presaging the dominance of knights in the Middle Ages of Europe.
All of these different armies were referred to, both in contemporary and modern documents, as “Legions”. It’s worth noting that there’s a pretty wide variance.
I agree that if you just tossed an NFL linebacker into a Roman legion of any era, he’s going to be torn to shreds in short order. He might have a better shot in the gladiatorial arena, really; that’s a deadly, bloody, and cruel game, but it’s still a game with rules, and modern athletes are really, really good at using those sorts of rules. Besides that, he won’t have to learn how to mesh his fighting with teammates, for the most part, and his massive size and strength will be a big asset against the single person or maybe pair that he’s facing.
If you gave the guy a few months to adapt, to learn how to handle the massive change in the sort of combat that he’s going to be fighting, I think he’d be adequate, with his lack of experience roughly balanced by his enormous size, strength and physical reflexes. The very best soldiers in the army could still whip him, I’m sure, but the Roman military machine did a very good job beating new recruits into fighting shape quickly, and an elite modern athlete will be perfectly placed to learn that training.
If a modern athlete grew up with the same sorts of culture that a Roman would experience, with the same sorts of deprivations and hardships, and still managed to grow to the same enormous size and strength, with the same extreme reflexes and physical proficiency, he’d likely be one of the greatest grunt soldiers in the history of the ancient world. We’d also have never heard of him. It’s pretty hard to distinguish yourself in the sorts of battles, where individual fighting is what gets you killed and carefully working with your squadmates is what gets the other guy killed instead. The closest thing to merit for individual actions that I can think of is the honor awarded for being the first man over the wall of an enemy town, at which our NFL player might excel until someone gutted him for trying.
All of that assumes the sorts of highly disciplined armies that you’d see in Rome, in Greece, in the Eastern states like Persia, and (I believe) in China and India. In a German or Gaullic army of the same era, our NFL player might fare a lot better. That sort of wild fighting, man-to-man instead of maniple-to-maniple, greatly favored men with massive physical size and strength for their ability to not just kill an opponent but terrify his friends while doing it. The Romans lost battles to the Gauls and the Germans, too, don’t forget, and when they did, it was usually because the barbarians managed to break the famous Roman formation and discipline, then maul the remaining Roman soldiers who were ill-equipped to fight outside of their tightly regimented system. Lots of the great Roman victories that we’re discussing in this thread where Roman legionaries held their formations and triumphed over a bunch of physically enormous barbarians were preceded by massive Roman losses, like the battles at Burdigala and Arausio where the Romans failed to keep their heads and did not present a united, disciplined front to the enemy.
In short, while the Romans frequently manage to defeat numerically and physically superior German tribes, those German tribe won some of the battles, too. There’s a reason that the Roman historians talk about how scary those giant Germans were, besides the fact that they’re glorifying their own side: sometimes being giant, terrifying, and individually gigantic won the Germans a battle.