Pompey manager Alan Perrin is likely to revert to his usual 4-4-2 formation with Diao and Silva both making their home debuts.
Serbian midfielder Zvonimir Vukic is also in contention to make his first Barclays Premiership start after impressing as a substitute in last weekend's 1-0 win at Everton.
John Viafara is battling for a recall in midfield while Vukic could also start. Linvoy Primus is still sidelined by a knee injury.
Birmingham have doubts over striker Emile Heskey (ankle) and right-back Mario Melchiot (knee) for tomorrow's clash with Portsmouth at Fratton Park. Midfielder Stephen Clemence is definitely missing with a torn hamstring.
Portsmouth squad: Ashdown, Priske, O'Brien, Stefanovic, Vignal, O'Neill, Viafara, Diao, Robert, LuaLua, Silva, Westerveld, Griffin, Hughes, Vukic, Karadas, Songo'o. Taylor, Mornar, Todorov.
Birmingham squad: Maik Taylor, Melchiot, Upson, Cunningham, Martin Taylor, Tebily, Clapham, Lazaridis, Johnson, Pennant, Butt, Izzet, Gray, Jarosik, Heskey, Forssell, Pandiani, Kuqi, Kilkenny, Vaesen.
