Sayer, 25, jarred his leg riding a challenge during the FA Cup preliminary round replay victory over Brighouse Town last week, and the club's worst fears have been realised.
A club statement said: "Potentially ending his season, Sayer has suffered cruciate knee ligament damage and begins his road to recovery immediately. He was at our game on Saturday – when we beat Hyde United 3-1 in the FA Cup First Qualifying Round – and will hopefully be a regular in the stands if you want to wish him well.
"Everyone at the club wishes Ben a speedy recovery and we hope to see you back, fighting fit as soon as possible."
Sayer has been with the club since 2014 and was the 2017 Northern League Player of the Year.
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