"JAG” ("Just A Guy") is a term that Bill Parcells used when coaching the Dallas Cowboys from 2003 to 2006. The term refers to a football player who is no different than any other football player and who doesn’t have the exceptional qualities to shine on the field on a consistent basis. “JAG” has been cited in print since at least March 2003.
A similar acronym is “RKG" ("Right Kind of Guy").
Wikipedia: Bill Parcells
Duane Charles “Bill” Parcells (born August 22, 1941) is a former American football head coach, most recently with the Dallas Cowboys from 2003 to 2006. He is known as “The Big Tuna”, a nickname about the shape of his physique derived from a team joke during his tenure as linebackers coach of the New England Patriots.
9 March 2003, Newsday (Long Island, NY), “A Bevy Of Big-Bucks Blunders In Beltway” By Bob Glauber, pg. B14:
There’s an expression NFL personnel people use to describe players like Noble: “Just a guy.” That’s JAG for short. Enough said.
20 April 2004, Dallas (TX) Morning News, “Parcellsisms” by Jean-Jacques Taylor:
PARCELLS TERM: JAG player
MEANING: A player who is just a guy—an average player who doesn’t matter in the big picture.
Posted: Sat Apr 02, 2005 10:52 pm Post subject:
He’s JAG (just a guy) as Parcells would say.
Best St. Louis Sports Arguments
By Bryan Burwell
Naperville, IL: Sourcebooks, Inc.
He was the ultimate JAG (“Just A Guy”), who played in the league yet never left a lasting footprint.
A Journey to the Center of the NFL Universe
By Rich Eisen
New York, NY: Thomas Dunne Books (St. Martin’s Press)
JAG: Just a guy.
Best days behind Moss?
By Jason Cole, Yahoo! Sports
April 29, 2007
In that same time span, Moss has become little more than a JAG. In scouting vernacular that’s a term for “Just A Guy,” a random player who is roster flotsam. That’s a bit of overstatement, but not by much.
14 August 2008, Pittsburgh (PA) Post-Gazette, “Fringe Steelers must seize chances where they can”:
Baker is JAG—scouting vernacular for “just a guy.”