Tuesday night's 3-0 defeat at Sunderland left the Royals with just nine points from their first 16 top-flight games and six points adrift of safety.
Injuries have not helped the promoted club's attempts to grab a foothold in the higher division, and McDermott is urging owner Anton Zingarevich to fund a winter recruitment drive.
He said: "The club made a decision in the summer - we didn't invest in the summer.
"We all know that January is a difficult window. We are fighting a battle here.
"That will be down to the owner to see where we are at. I believe we need to do that.
"We are in such a stronger position financially, whatever the outcome of this season is, than we were two years ago, a year ago.
"We want to get out of this, so we have to do everything we can in our power to get out of it, and if that includes January, my view is that we have to do what we have got to do."