Merson cannot play on-loan keeper Joe Murphy in Saturday's FA Cup clash with Merthyr Tydfil as his parent club Sunderland will not allow him to become cup-tied.
And regular shotstopper Andy Oakes has hand ligament damage and on-loan replacement, meaning 18-year-old shotstopper Rene Gilmartin will play.
Merson said: "Mick McCarthy doesn’t want Joe cup-tied and I totally understand that.
"It’s not an ideal situation for us though and that’s why we were looking for a keeper without a club to come in and play a one-off game for us.
"So I contacted David’s agent but unfortunately the insurance he took out when he retired means he can’t play again."