"If Portsmouth choose to select a reserve team that's up to them," said the Scot.
"We wouldn't like it but that's the way it's panned out and we'd have to accept that. But Portsmouth are capable of winning even with their reserves."
Birmingham must prevail against Blackburn at St Andrews next Sunday to have any chance of surviving and McLeish insists they will leave nothing in the locker room.
"We have to go out to win. We have nothing to lose and must throw caution to the wind," he said.