McNamara had just fired off a handball in Sunday's 51-point win when Heads ran in and collected the midfielder front-on with her shoulder.
A dazed McNamara had blood coming out from her face as she was assisted from the ground.
The 24-year-old was taken to hospital, with the Demons confirming on Monday she has undergone surgery, with no timeline set on her return.
The incident was classified as careless conduct, severe impact and high contact, drawing a three-match ban.
That equates to a quarter of the 12-round home-and-away season, and Heads will miss games against Sydney, the Western Bulldogs and Adelaide unless Port can downgrade the charge.
In other tribunal news, West Coast's Abbygail Bushby was handed a one-match ban for her clumsy collision with the Brisbane Lions' Sophie Peters.
Peters had her head over the ball when Bushby ran in and tumbled over her opponent.
Richmond's Montana McKinnon was handed a one-match, rough conduct ban for her run-in with St Kilda's Jesse Tawhiao-Wardlaw.
Sunday's win over the Saints was only McKinnon's third match for the Tigers across the past three years, following on from two knee reconstructions.
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