Mcguinness is not half the star and draw that Nakamura is. Saying that WWE won't allow one of the biggest stars in the world, no time to recover, sounds pretty unrealistic.
Mcguinness was supposed to join WWE at the same time as Bryan, and McGuinness was the guy they actually wanted of the two. They had big plans for him (basically throw Barrett to the side, build the first season of NXT around McGuinness and push him to the moon on the main roster). There was legitimate talk of making him the first British world champion. But he had a Biceps tear that never healed proper that basically threw him off the WWE radar and ultimately lead him to the end of his career.
If Nakamura has some form of injury that their doctors will not clear, he won't be signed. Because WWE does not want to sign someone that comes in with an injury, then re-injures himself immediately and that they then has to pay for.
If Nakamura has an injury his eventual signing could be delayed by months, or possibly pulled all together. Look no further than your own fav tag team TMDK. They were supposed to join WWE last June, but Haste had a knee problem that threw a big spanner in that.