The 20-year-old will now pick-up a three-match ban the Magpies can ill-afford, with the squad already depleted by injuries' and suspensions to Julien Baudet and Steve Scoffham, who will return from his ban on Saturday at Maccesfield.
And Thordarson had little sympathy for McMahon, whose act of idiocy helped compund an already miserable night for the travelling support.
"I apologised to the lad and shook his hand, because it is not in my nature to play the game in that manner.
"He will be dealt with, and he will be fined. It was very disappointing, because I thought the game was played in a fair and honest manner."
Thordarson, whose experiment with a 3-4-3 formation badly back-fired, also had to contend with an injury to David Pipe, though the initial diagnosis is that it is not as serious as first feared.