Former Cowboys 1st-round pick Marcus Spears is putting Dak Prescott ON BLAST over his quarantine party ... saying, "This was one of the most disrespectful things I've seen in a long time."
We broke the story ... Dak played host to a big gathering at his place in Prosper, Texas over the weekend -- and one witness says there were as many as 30 people in attendance at the height of the festivities.
Of course, this is a huge no-no ... as government officials have urged self-isolation amid the coronavirus pandemic -- and Spears ripped the Cowboys QB on Monday for putting the party on.