Steven Avery Prosecutor: A New Trial Should Be Granted, IF ...

Ken Kratz, the prosecutor who sent "Making a Murderer" defendant Steven Avery to prison for life, says if scientists develop a test showing the incriminating blood came from a vial, he should get a new trial.

Kratz appeared Monday on "TMZ Live," and we asked him about the defense argument that cops took Avery's blood from a vile and planted it at the murder scene. Kratz opened the prison door just a crack.

We also asked him about a shocking comment he made to the jury during closing arguments ... that EVEN if cops did plant evidence, there was plenty of other, legitimate evidence to convict Avery.