Fisher hurt his knee at home and has had a series of injections over the summer to ease it.
Boro told Neil Warnock told BBC Tees Sport: "We don't actually know yet and we're crossing our fingers.
"It was an accidental thing. I think he had his legs up at home and I think someone in the family just fell on his knee and just clinked it.
"We don't know yet. He's had three injections over the last few weeks and we should know more in about a week's time.
"But we think he could be out for a while at the moment and we're just crossing our fingers that it's nothing worse."
North End sold Fisher to Boro in the January transfer window.
The 27-year-old made 119 appearances for PNE after signing from Rotherham in July 2017.
Fisher has played 12 times for Boro, one of those games against North End in March at the Riverside Stadium.
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