Ex-NFL Lineman Justin Bannan

Ex-NFLer Justin Bannan has pleaded not guilty by reason of insanity in his attempted murder case ... after the defensive lineman allegedly shot a woman back in 2019.

41-year-old Bannan -- who played 11 seasons in the NFL -- was arrested and charged with attempted first-degree murder, assault and burglary back in October 2019 ... after he shot a woman in the shoulder in Boulder, Colorado.

The woman survived -- and she's suing Bannan over the shooting -- but he's also being prosecuted criminally and faces serious prison time if convicted.

Officials say at the time of his arrest, Bannan told a bizarre story about how he was on the run from the Russian mafia and had ditched his cell phone over…