Read all the tags. Nothing stood out to me.
The size tag at the bottom says "Engineered by Reebok to the exact specifications of the National Football League" and has the NFLPA logo on it as well as the NFL Equipment shield. Washing instructions are normal gramatically as well, "Machine wash cold separately, do not bleach, tumble dry low heat, remove promptly, do not iron"
Only thing I can see that might make it a knockoff would be the "made in Korea" but then again NFL jerseys are a Reebok product so they may very well all be made in Korea. or Thailand. Or anywhere you can pay someone 25 cents a day.
Regardless of whether or not it's a knockoff, it looks great and it was in my price range, so I don't care either way. I'm not a jersey collector of any kind, I just needed a Bills jersey.