By Jen Heger - Breaking Celebrity News Legal Editor

Dr. Conrad Murray's criminal defense team has been dealt a crushing blow when the judge presiding over his involuntary manslaughter trial, ruled that Michael Jackson's dermatologist and long time friend, Dr. Arnold Klein won't be allowed to testify.

Dr. Murray wasn't in court for the pre-trial hearing on Monday in Los Angeles.

Judge Michael Pastor agreed with the Los Angeles County District Attorney's Office -- which opposed the defense's attempt to call Dr. Klein as a witness in the trial.

Judge Pastor stated that Dr. Klein's testimony isn't relevant to the case, and that it could confuse the jury.

“Dr. Murray's lawyers were planning on asking Dr. Klein about Demerol that he allegedly prescribed to Michael Jackson in the days before he died. The defense was going to contend that Dr. Murray didn't know that Dr. Klein was giving Michael Jackson Demerol, at the same time he was giving him Propofol, to sleep," a source close to the case tells Breaking Celebrity News.