Two things first:
- NFL players are frakking huge. The biggest person I’ve seen in real life was a quarterback at a local university. He brought his great dane in to my family’s dog grooming shop. The dog came up to my waist, it was just above this guy’s knee. He was about two of me wide. I’m right smack in the middle of the height bell curve. Quarterbacks are often the smallest guys on the field. I looked like a little kid next to him.
NFL players are fast. Even those big fat dudes on the front line can accelerate like Olympic sprinters. Most of them have absurdly fast reflexes too.
Remember that these guys are selected from a few thousand of the hopefuls that try out from high school and college teams over the entire US. Remember how the star player at your high school was huge and talented and made even most of the other players look slow and awkward? He’s the small fish that got thrown back before he even made the draft.
- Depending on when and where you’re talking about (not all time periods and locations are the same), the people would probably be smaller than the average for now, but would probably be stronger for their size and more able to withstand hard work and deprivation than modern people.
The guys who usually ended up fighting in medieval times were two tiers: levies, and landed gentry on up. Knights and above were bigger than most of the peasants and yeomen due to better nutrition. They did stuff for fun that was basically warfare-lite: hunting, riding, etc. They were, for all intents and purposes, the medieval equivalent of modern professional athletes. They knew their weapons and equipment inside out. Most of them could kick the living shit out of even our MMA fighters because they trained to kill and maim people, not just beat them up, and they were probably about as well conditioned — if not as specialized — as a modern dedicated fighter.
Levy troops were semi-conscripts. Participation in military training and action was more or less a kind of tax. Members ranged from peons to yeomen / free farmers. They would have had much less training, poor equipment, and a wider range of health and conditioning. But even so they probably weren’t starveling wretches, or they would have been rejected for service and someone else taken in their place. All of them would have experience with rough and tumble fighting, and some even actual life and death fights with criminals, bandits, or wild animals.
In ancient times (Babylonian, Greek, Phoenician etc. armies) there were no full-time professional soldiers outside of a handful of nobility. Just about everyone put in some time in the military, particularly the “middle class” equivalent landed non-nobility, which made up the central hoplite corps upon which most Greek tactics depended. Most of those philosophers you read about, Plato, Socrates, Aristotle, were veterans of several campaigns, which means they were pretty much badasses. Nutrition and lifestyle were pretty good too, better than most times and places in medieval Europe, actually.
Roman soldiers, however, were professionals. They served up to 20 years. They fought in strict formation, and carried a bunch of gear. Anyone capable of humping 30–40 kg daily over an average of 20–30 km a day, and then setting up camp with a palisade and trenches before bedding down for the night is someone capable of kicking most modern men’s asses even without special training.
They were trained well, and fed pretty well too. There’s a reason besides excellent logistics that Roman armies steamrolled over half the civilized world; their training, discipline, and procedures were insanely good compared to their foes. The Germanic armies in particular were noted as fighting heroically, and the stature of many of the men was described as being greater than any individual Roman, but they still got the shit kicked out of them by the little dark buggers.
So, how would an NFL player fare as a warrior?
Advantages: Size; strength; good starting health (maybe, depending on injuries); superior reflexes; should be a very fast learner of physical skills.
Disadvantage: Size; calorie requirements; relatively low endurance and tolerance to hardship; no skills with the weapons and armor of whatever time and place he ends up.
Aside from single combat, being a big bugger isn’t much of an advantage in the kind of warfare we’re talking about. Most of the men in the army might have been training, even sporadically, with the weapons since they were kids. And that’s the levy troops in medieval times, or the bulk of the ancient Greek and Romans. The knights and nobles would have been doing it since they were old enough to walk, and training for war as a more or less full time job since they hit puberty. His physical attributes wouldn’t offset his lack of skill at first. Even having a strong talent for learning physical skills wouldn’t help much when he’s years behind the curve.
You might stick him in the middle of a formation, once you get him kind of trained, but chances are he’d get killed in the first couple of engagements. Worse, he might panic and run, ruining the morale of your infantry and leading to a rout. He’s too frigging big to really support his shield mates well, so it might be better to stick him on the end of a line and hope he doesn’t fuck things up too badly.
The biggest limiting factor is that he would have no experience with the mindset of ancient warfare. Even gangbangers who grew up getting shot at in their 'hood have next to no experience with getting up close and personal. Infantry combat in ancient and medieval war meant walking forward in formation while arrows or other missiles rain down around you and on you, and every so often some poor bugger gets one in a gap in his armor or through an opening in the shield wall, and he goes down messily dead or seriously wounded.
It means being shoved up so close to your enemy when the lines make contact that you smell his bad breath and slip in his shitty guts after stabbing him to death and being soaked in his blood, while several hundred of his friends in the crowd try to do the same to you. The men behind you threaten to trample you underfoot if you don’t keep pushing forward, and a group of noble arsewipes charge by on horses that seem half the size of an elephant from your perspective, not really giving half a fuck if they squish you or some poor bastard on the other side.
The closest modern equivalent I can think of would be like being in the most violent mosh pit you’ve ever encountered at a concert, except everyone is armored and beating on you with sharp, pointy and/or crushing things, with occasional heavy equipment rolling by at full speed and equipment raining down from the scaffolding. A foul weather game in the winter in Minnesota might feel like a vacation on a beach in Bora-Bora by comparison.
Honestly, if he was actually accepted into the army, I’d be shocked if he lived past the first battle. If he did survive a year or so he’d be somewhat of a veteran and would probably be as likely to live as any other soldier. But there’s no way he would be considered a valuable warrior for a loooooong time. Unless, of course, he happened to have taken a degree in medieval history, practical chemistry, metallurgy, or some other useful field at university; knowledge is always very valuable.