• Monday was day six in former Minnesota police officer Derek Chauvin's murder trial.
• Minneapolis Police Chief Medaria Arradondo testified Chauvin violated policy when he pinned George Floyd's neck under his knee for more than nine minutes. 
• This was "another nail in the coffin" for Chauvin's defense, Civil Rights lawyer David Henderson said.

Testimony from Minneapolis Police Chief Medaria Arradondo was "another nail in the coffin" for former Minnesota police officer Derek Chauvin's defense, Civil Rights lawyer David Henderson said Monday.

"The fact that he is a key witness is an understatement. ... The window for the defense to mount an effective presentation for this jury is quickly closing, if it hasn't already shut, with the chief's testimony," the former prosecutor told CNBC's "The News with Shepard Smith."

Monday was day six in Chauvin's murder trial. Arradondo testified that Chauvin violated policy when he pinned George Floyd's neck under his knee for more than nine minutes. 

"Once there was no longer any resistance, and clearly when Mr. Floyd was no longer responsive and even motionless, to continue to apply that level of force to a person proned out, handcuffed behind their back, that in no way shape or form is anything that is by policy, part of our training and is certainly not part of our ethics or values," Arradondo said.

The police chief said he did not observe Floyd actively resisting in the cell phone video of his arrest, and he couldn't even tell if Floyd was still alive at a certain point. 

Henderson, a CNBC Contributor, explained that Chauvin's defense missed the opportunity to make two important points while the police chief was on the stand.

"They should have demonstrated that problems with policing are systemic, not individual, and to the extent that Derek Chauvin has been painted as a lone wolf, they should have made the point that lone wolves don't roam in packs," Henderson said. 

The former prosecutor told host Shepard Smith that the defense also missed two additional opportunities during Monday's trial. First, he said that Chauvin's witness chair should not have been removed from the court, because it would impact the jury's opinion of him. 

"You should have someone sitting there representing him, showing that they care, and if you don't immediately have someone, you had a year to help him develop a relationship for a person to come to court with him," Henderson said. 

Second, Henderson also suggested that Chauvin should have taken the stand Monday, in the context of an orchestrated trial strategy.

"This should have been like Jack Nicholson taking the stand in 'A Few Good Men,' it really should have been, and since it wasn't, you are going to have to find some other opportunity that may present itself that makes sense for him to take the stand afterwards," Henderson said. 

Chauvin's defense team did not immediately respond to CNBC's request for comment.