Kevin Costner -- Dumbest Fight Ever ... For Credit on Sucky 'Draft Day'?!

Kevin Costner's movie, "Draft Day" was a total bomb in theaters -- but amazingly there are people actually fighting to claim credit for the box office disaster.

Lionsgate Films filed court docs trying to head-off a possible legal challenge by William (Wil) Mara, whose novel "The Draft" covers the same subject as the movie ... the NFL draft.

Mara claims Lionsgate jacked his book for their movie -- even though the studio touted the film as an original piece of work ... according to the docs obtained by TMZ.

LG claims the book and movie differ substantially -- different teams, plots and characters -- and the entire movie takes place on actual draft day, and most of the book does not. Mind you ... the studio's bragging about a movie that made a mere $29 mil.

Lionsgate wants this sorted out before 'DD' comes out on DVD -- which ... it's assuming people will buy?