The 31-year-old, who picked up the injury against Doncaster Rovers last month, will now likely miss his side's final four games of the campaign, the club's official website has revealed.
Reid had been a key figure for the Reds before sustaining the injury, scoring 10 times in Forest's playoff charge.
"I cannot tell you how disappointed I am to have picked up another injury," the Irishman said in his matchday programme notes. "It's been a very frustrating period for me but thankfully it isn't serious and I'll be ready to go again next season."
Forest face Birmingham City later today, before closing their season with games against Leeds United, Bournemouth and Brighton & Hove Albion.