The doctor who declared Michael Jackson's death a homicide claims it's extremely unlikely the singer killed himself with a deadly dose of Propofol -- insisting Dr. Conrad Murray is more likely to blame.

Dr. Christopher Rogers just testified -- Murray's description of the final moments leading up to Michael's cardiac arrest just doesn't add up.

Murray claims he left for two minutes to go to the bathroom -- and when he returned, Michael wasn't breathing. The defense will argue MJ self-administered a deadly dose of Propofol in that time.

But Rogers insists ... Murray's scenario is nearly impossible -- because in that two minute window, MJ would have had to wake up (under the influence of several powerful sedatives) and skillfully administer the deadly dose ... which would then have had to travel to his brain, causing him to stop breathing.

What's more likely, according to Rogers -- Murray administered several doses of Propofol in the hours leading to Michael's death without properly measuring them out ... resulting in an overdose.