The 33 year-old forward, who has scored nine goals in League Two this season, has not featured for the Magpies since injuring his ankle against Crewe Alexandra on October 15.
"I spoke to Steady about two days ago," Sheridan told the Nottingham Post.
"I've not spoken to the physio regarding the results of the scan.
"I would have expected that if they had the results back they would have come and told me, but he's still a bit sore.
"I don't like putting timescales on things, but the physio says we are looking at six weeks, then it's a case of diagnosis and how he is healing.
"We are probably looking towards Christmas probably.
"It's very disappointing as a team, and for him as a lad, because he was scoring goals.
"He probably would have scored a couple more in the time he has been out so it's a big downer."
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