The Braveheart star has been at the centre of a media storm since his split from ex-girlfriend Oksana Grigorieva, the mother of his young daughter, Lucia, turned nasty.
The Russian singer has accused her former partner of beating her and knocking out her teeth, while taped phone conversations appear to feature the movie star hurling abuse at his lover in a series of venomous rants.
But Campbell, who directed Gibson in recent thriller Edge Of Darkness, is adamant the negative headlines plaguing the actor are just "sad".
Speaking at the Comic-Con convention in San Diego, California, he tells the AAP, "I think Mel is a brilliant actor. I think Mel is a terrific guy. I just think it is sad. I just wish him the best."