The club captain has not played at all this season after breaking four bones in a hand during the pre-season training camp in Ireland.
He missed a behind-closed-doors friendly on Tuesday, which had been arranged to give him some match fitness, with a calf problem. Last week he missed the reserve game at York, with what the club said was a groin strain.
Reid could still find himself partnering Niall Canavan at the County Ground on Saturday, with David Mirfin serving a one-match suspension.
"[There is] no conspiracy, groin strain then managed to hurt calf while trying to keep fitness up. Probably due to missing pre-season," he wrote on Twitter.
"Disappointed but not despondent, was always a possibility after missing pre-season and working extra hard to catch up.
"Hope to train Thurs and be available for at. Desperate to try and help the team."