How about we flip the question: How would an ancient Roman warrior transported to a modern army fare? It might serve to illuminate some of the blind spots you guys have in thinking that a modern man, however big and strong and physically gifted, would have in dealing with an Iron Age society.
We know he’s tough. He shouldn’t have to do any of the survival training since his life was a lot rougher than anything our modern forces face, roughly equivalent to a SERE course, except that in his time there was no Geneva Convention, so he would expect to get tortured and/or crucified if captured. Rather a bit more in the way of motivation there.
He’d be disgusted at the relative lack of discipline in marching and formations, since his life literally depended on everyone being in position properly in response to dozens of different flag and drum signals. If some idiot made him go through boot camp, I would expect to find some of his squadmates beaten to bloody shit before they learned that the little guy doesn’t have an off switch. When all fighting is hand-to-hand, and there are basically no cops, cultural training usually defaults to going apeshit immediately instead of a gradual ramp-up in violence. And he’d be a trained veteran killer, so even someone the size of Mr Hypothetical NFL Player would probably be incapacitated or maimed in seconds.
Punished with push ups or latrine duty? <pffft> Please. You wouldn’t get his attention with anything less than severe corporal punishment, and he’d probably treat you with contempt for not being a real officer by meting it out immediately and brutally for infractions.
What does he lack? Any form of training in firearms, explosives, and equipment. He’d be right at home in CQB style; kicking in doors, loaded down with armor and gear, but you’d have to get him trained with firearms first. Archery is very vaguely similar, but not really, and the form of training he’d be familiar with wouldn’t work with modern tactics and weapons.
How long before he’d be a valued soldier? If you didn’t have to throw him in the brig permanently to protect the rest of the soldiers sometime in the first few months, he’d probably make a good little headbreaker in a year or two. He would be far better at some things than the usual raw recruit the Army has to deal with, but would have some educational lacks that even high school dropouts don’t in our society.
He’d have to learn to shoot — which is honestly darn easy since we’ve striven to make firearms very user-friendly and have gotten pretty good at training even complete idiots to use them. Compared to sword, pilium, shield training, it would be a snap. You’d have to undo his training to stand steady under fire, but since that goes against natural instincts more than taking cover, that probably wouldn’t be too hard.
He’d have to get some proficiency in English or whatever local language he ended up with, which would be pretty easy for him since being monolingual was extremely rare anywhere in the ancient world. I wouldn’t bet on him making a good driver, and wouldn’t trust him with complicated machinery. He’d be hopeless with computers, probably even with extensive training, unless everything was standard enough to learn by rote.
He probably would never be more than functionally illiterate if he wasn’t a reader already — and he probably wasn’t. (Contrast this with his trained reliance on memory, and Mr NFL in Ancient Rome would be screwed without writing tools.) That would completely bar him from any kind of special forces or any kind of rank.
He would be a constant problem when interacting with civilians since the cultural standards were completely different from any modern industrial society. He would probably be a discipline problem in the military unless he learned to respect his commanding officer despite the behavioral softness compared to Roman standards.