McMahon underwent surgery in Altnagelvin Hospital in Derry last night on a double fracture.
The injury occurred in the opening league game of the season, in which his club Omagh was away to Moortown.
McMahon had just recently returned to action after a lengthy recovery period from a hamstring problem.
He had been deputising at full back in the latter part of Tyrone’s NFL campaign for his brother Justin, who also has suffered a setback in his recovery from a hamstring injury.
Meanwhile Enda McGinley was stretchered off with only two minutes left in Errigal Ciaran’s win over Cookstown at the weekend.
It was only his first game back since injuring his ankle in Tyrone’s National League win away to Sligo in February and it’s feared he has hurt the same ankle again.