There are two glaring issues with what you've said here.
1. The size requirements needed to play certain positions in the NFL mean the pool of humans being to choose from to fill those spots is quite obviously MUCH smaller than it is to play soccer (any position in soccer). So yes Stephen Neal or Antonio Gates or Jimmy Graham were able to transition over to football and have success. But they never would have even gotten the chance if they literally weren't in the top 1% of human beings from the combination of athleticism PLUS sheer size. You can't do what they did just "being a great athlete coming from another sport". You have to be that PLUS have physical size that 99% of humans don't possess. Keep in mind the guy that Neal beat for his NCAA wrestling title (Brock Lesnar) also tried to make an NFL roster and could not despite being a genetic freak. So guys like Neal are extremely rare. There have been a few (I mentioned 2 others), but that's it.
2. So piggybacking off point #1, we were talking about soccer players transitioning to the NFL. No soccer players that I know of possess the sheer size needed to play the specialized positions that you are referencing. Soccer players are skilled athletes like WR's, RB's, DB's, etc. in the NFL. Those are the only positions where professional soccer players would have the necessary size to be able to play. And no soccer player would ever be able to just pick up the skills needed to step into the NFL and play one of these positions well.
So while Tyreek Hill (for example) is likely faster than any soccer player on earth (assuming Bolt is done with soccer LOL), he couldn't step onto a pitch and compete in the EPL after practicing for a year or whatever obviously. But nor could Neymar try to learn to play WR for a year or two and make an NFL roster. And no soccer players could play on the line or TE because they are just too small.