Pearson had a scan on Sunday and as expected that showed ligament damage in his left ankle.
Speaking on a Zoom press conference on Monday morning, Neil did not put an exact timescale on how long the 25-year-old would be out.
However, Pearson won't require surgery, with the ligament able to heal naturally.
Neil said: "It is about the conservative management of it.
"Ben doesn't requite an operation but it is how quickly it will heal, everyone is different in that sense.. We will have to wait and see.
"I don't really want to commit to a period of time at the moment."
Pearson suffered the injury after half an hour of the 2-2 draw with Wycombe at Deepdale.
There was no one near him when he attempted to change direction to make a pass and his foot slipped, causing him to twist.
He had only recently returned from a six-week lay-off with a groin injury, the Wycombe game being his third start since coming back.
It seems likely this absence will be measured in several weeks, running into the new year.
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