McNamara was substituted after a horror head clash on Sunday and seven-day concussion protocols are expected to draw a line through the luckless Matildas striker if City make the decider against Melbourne Victory.
The league's brightest star, McNamara was back in City's starting line-up at Casey Fields after coming off the bench in last week's away semi-final leg in Gosford, which was a 2-2 draw.
The 22-year-old was showing dangerous form but her match came to an early end in the 10th minute when she went up for a header against Mariners goalkeeper Sarah Langman and the pair heavily clashed heads.
Both hit the deck, with McNamara staying down requiring treatment.
The distraught striker slowly walked off the field and was quickly ruled out of the game, with Shelby McMahon replacing her.
If City qualify, the grand final will take place next Saturday, as they are due to subsequently fly out to China for their Women's Asian Champions League semi-final.
Given the six-day turnaround, McNamara would be unable to play in the grand final.
McNamara has had an extraordinary season, scoring 15 goals in 18 games after returning from her third career knee reconstruction.
Sunday's game could also have been her final ALW appearance, as McNamara has attracted attention from overseas.
City's job was made harder when winger Lourdes Bosch received a straight red card in the 25th minute for needlessly hitting Central Coast's Brooke Nunn in the face after the Mariners attacker had pushed her.
