Birmingham have learned that youngster Jordon Mutch will be out for three months with a broken foot while midfielder Morgaro Gomis and strikers Cameron Jerome, Marlon King and Nikola Zigic all remain out.
New loan signing Guirane N'Daw is ineligible while defender Scott Dann, who has been linked with a move before the upcoming transfer deadline, is also sidelined.
Boss Chris Hughton insists his absence does not spare him a difficult decision over whether to allow him to become cup-tied or not.
He said: "He has a slight injury and there aren't any outside reasons why I would leave him out.
"If I had thought it was right to have played him, I would have, but he has a slight injury."
Zigic and Liam Ridgewell have also been linked with other clubs but Hughton added: "We have had lots of interest and enquiries but we are no further down the line today than last week or the week before."
Hughton has admitted he would be interested in signing forward Nile Ranger from his old club Newcastle but has given no indication that a bid could be made.
He said: "It is just enquiries and finding out where he is at this particular moment, but at this stage of the season there are numerous players we could be making enquiries about."
Hughton's immediate focus is on Nacional and making progress in Europe.
"It is certainly going to be a tough game," he said. "Nacional showed enough in the first leg of why they are in the competition.
"They are a very offensive side who changed at half-time and went a little bit more offensive.
"I would expect a very difficult game. They are a good side on the counter-attack and have some good technical ability."