Mellon was forced into a reshuffle at the back 15 minutes into Saturday's 2-2 draw at Cheltenham after skipper Steve McNulty limped out of the action.
Additionally, Blackpool youngster Ashley Eastham's decision to join Notts County in favour of extending his loan with the Cod Army has left Mellon short at centre-back, but the Scot expects to bring in an unnamed defender by Thursday.
"We're looking actively to try and bring in another centre-half because we only have one recognised centre-half at the minute and that's Youl Mawene," he said.
"We'd hoped that we'd have Ashley Eastham for another month because he'd agreed to come, so we allowed Rob Atkinson to go to Accrington. But then Ashley changed his mind and it left us a wee bit bare to be honest.
"But we've got a good scouting system and good contacts and we think that we've hopefully secured somebody who will come in and start training with us on Thursday."