If you got Ladanian Tomlinson and a few of his closest friends...

[Turkish Mickey]

the nfl guys wouldn't come close. (tldr - coles notes at bottom)

i played rugby (and football) in high school in canada, where nobody picks up the game until grade 9. in my senior year we played a team from wales comprised of grade 9's and 10's who were on tour. our team was mostly grade 12's with some 11's. at that age, two years makes a huge difference, and we were way bigger, stronger, faster etc. than them.

they absolutely dominated us. they'd played the game their entire lives, and despite the fact that we were clearly athletically superior we couldn't even come close. they've played the game they're entire lives, their technique was way better, their understanding of strategy, their set plays, they were miles ahead of us in every aspect.

the nfl guys wouldn't come close in 6 months. just the "remodeling" of their bodies would take a while, in the nfl you go hard for 7 seconds and then you get 30 seconds rest. and that's just when your position is playing. often you see returners who run for 60 or 70 yards on a big play head to the sideline for oxygen. you think that guys going to be ready to play effectively for 80 mins straight anytime soon? just to turn nfl athletes into rugby style althletes is going to take a while.

also, the nfl squad is going to be severly lack in specific skill sets. kicking is perhaps the most obvious area, but passing, rucks, scrums, everything.

the nfl team would, given enough time (i have no idea how much this is), probably be able to produce part of a world class team. but in certain key positions, such as scrum half or fly half, i don't think you could take a fully grown adult and transform him into a world class player. feel and understanding for the game, as well as rugby specific skills, are far too important for these positions.

the positions where i don't think the nfl could produce top players even if they trained for years (let alone six months) are some of the most crucial, and without top level players at these spots it would be extremely tough for the nfl club to win against a world class team. so when people post that the nfl wouldn't win if you gave them 5 years, i'm not sure it's true, but it's not in any way unrealistic.

coles notes - in my (actual, real life) experiece, being way bigger, stronger and faster is not enough to beat a far more experieced rugby team. (or even keep the game close)