Either it is a derivation or not. If an art piece is a derivation of an other art piece, will eventually be settled at court, even if every single pixel has an other value or hue and has been scaled down or up

...in my current project is derivative work done by our previous artist.

Yelling this in public forums is e.g. a bad start for your side...

Tips:

1. Ask the previous artist to use his art (demand a license).

2. Ask a lawyer.

3. Start over again and make your own art.

Always make decisions on the defensive, as if not only your legal standing is on the line but the survival of the organization. You are far better for having started art on your own from the start rather than using others work even if by permission.

Have I used other's work by permission? Sure, but I have been putting demand on myself in the last couple years or so to create 100% original art content unless my boss insists on using licensed work. This forces me to improve both the speed of creation and my art skills while being totally safe legally: 3 huge reasons to make original content for games.

It becomes clear with forethought that doing the absolute right thing is harder but the rewards are greatest long term.

Clinton