-soo suffix question

Hi everyone,

I'm studying on my book but i have a doubt about the -soo suffix when it's used with "sa" -sasoo.

The book says: "With ii(yoi),nai and the present negative forms of both i-adjectives and the copula,sa is inserted before -soo."

Then in one example it's wrote:
"Kizu wa itaku nasasoo deshita" -> The wound semed not to be painful.
It shouldn't be something like "Kizu wa itasasoo nai deshita"?
The same in "Ano isha wa joozu ja nasasoo desu".

Does it mean that with negative form of i-adj only nai take the -sasoo suffix while the negative i-adj remain unchanged?

Thanks.