What makes it likely to actually be true, IMHO, is that they cut Johnson in the first place. It's not like they got any NEW information on Thomas' potential since that. Bringing in someone you just cut makes a lot more sense if the reason was simply an injury.
Preface - This is all assumption on my part.
I was kinda thinking they didn't play Johnson much in preseason to keep him under the radar a bit and planned on letting him go to hopefully sign him after week one. In case someone doesn't know, when you sign a vested veteran after week one their salary isn't guaranteed but if they are on your roster for week one it is guaranteed.
So I also agree that the injury is likely true. Even if my assumption is correct, they obviously didn't feel like they had the luxury of waiting it out.
This is also a statement about Hilliard. You have to conclude that this staff only sees him as a last resort running back with spot duty at full back and primarily a special teams player. How long does a guy like that stick around?