Dr. Conrad Murray believes his testimony in Michael Jackson's wrongful death lawsuit would lead to catastrophic consequences for one side -- and he's not saying which -- so it's best to leave him the hell alone.

Murray made the veiled threat last night in a bizarre, rambling voice mail he left on a friend's machine -- obtained by TMZ -- and you have to hear it to believe it.

The doc who's serving time for manslaughter says, "If I am compelled to speak [in court] I can cause one party or the other to experience the impact of an immediate seismic shock" resulting in "a total and complete debacle" for both sides.

Murray -- who was subpoenaed to testify in Katherine Jackson's $40 billion lawsuit against AEG -- has pled the 5th because he doesn't want to incriminate himself in any upcoming legal proceedings stemming from his appeal to overturn his manslaughter conviction.

Which begs the question ... what's he NOT saying? You gotta listen to this thing.