Grayson also lost full-back Stephen Crainey before Tuesday's 2-1 Carling Cup victory over Southend with a hamstring injury.
"He's had a scan on his knee and his shin and unfortunately Evo's going to be out for two to three months," Grayson said.
"We are always looking but it might force the issue now with Evo being out and we'll see how Stephen Crainey is and if his hamstring's bad then we're basically down to five defenders.
"We will see how everybody is and it might be a case of getting a couple of players in before the weekend but dependent on how the lads are on Thursday."