James Harrison did it without a problem.

I like that the example goes for NFL players unable to compete in rugby, but there are few-if-any rugby players that could make or break NFL tackles, either. There are few if any rugby players who could make or shake NFL blocks. Football is a game of specialization.

Now turn around and ask me if a top-tier Wide Receiver, Corner, Safety or Running Back (the positions most similar in physical demands to rugby) could play rugby effectively. Athletically, they'd likely be fine. Position-wise, mentally, they wouldn't.

I mean I think it's funny that there's this talk of injury resilience in rugby, and "toughness," when the worst injuries in football are significantly worse. Google: tyrone prothro injury. Besides, there are plenty of "tough" football players who do play through injuries (Herschel Walker did it famously by putting a separated shoulder back into its socket and playing the next series, as well as Mike Vick, a quarterback playing a full season for the Falcons with both of his shoulders separated.)

Also, it's worth mentioning that rugby players do have a certain toughness in that they don't care about what they look like, which is why they choose not to wear helmets. Helmets to protect the face and eyes (and, later, cranium) are the reason football players wear pads; tacklers lead with their helmets (how to stick a tackle? put your helmet on the ball.) Football players aren't willing to sacrifice their futures to prove they are some form of "masculine." If that's your definition of toughness (not ability or willingness to endure physical pain but willingness to accept unnecessary scarring as a result) then I guess you're right. But that definition is awfully convenient for a rugby argument and awfully worthless for any other application.