Derek Chauvin Prosecution's Star Witness Says OD Didn't Kill George Floyd

The prosecution in the Derek Chauvin murder trial is trying to poke major holes in his defense ... with a doctor's powerful testimony that any healthy person would have died if treated like George Floyd.

Dr. Martin Tobin, a physician specializing in pulmonology and critical care, took the stand Thursday and ripped into the heart of Chauvin's defense -- that Floyd allegedly died from a drug overdose and not by Chauvin's knee on his neck.

The doc goes into pretty deep detail on why, in his expert opinion, a healthy person would not have been able to survive a knee to the neck for 9 1/2 minutes.

Dr. Tobin testified that, by his calculations, Floyd lost all of the oxygen in his body about 3 minutes BEFORE Chauvin lifted his knee off George's neck. Tobin's being called the prosecution's star witness, as his testimony's been one of the most powerful moments in the courtroom, so far.

The doc went on to say George died from a low level of oxygen causing his heart to stop, essentially attributing his death to him being handcuffed face down on the pavement with Chauvin's knee pinned to his neck.

Remember ... Chauvin's defense attorney Eric Nelson told the jury last week Floyd "put drugs in his mouth" when confronted by cops and said the autopsy found meth and fentanyl in his system.

But, Dr. Tobin's testimony is clearly the prosecution's attempt to blunt that OD theory.