Fisher, who PNE sold to Middlesbrough in January for a reported £300,000 fee, suffered a freak knee injury at home during the summer.
Boro manager Neil Warnock said: "We’re not sure at the moment but it doesn’t look very good,”
"I’ll know tomorrow but it’s looking like an operation and you’re talking nine months so we’re not optimistic about that.
"From what he’s telling us it was a complete accident the way that he did it, with a mate slipping and falling on him and crashing onto his knee.
"It’s the back of his knee.”
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"I’ll know tomorrow but it’s looking like an operation and you’re talking nine months so we’re not optimistic about that.