"It is good to beat United but we have to stay focused and make sure we win the competition now," said the ex-Chelsea man. "Of course, we can win the FA Cup. If we can beat Manchester United, we can win the competition.
"It would be a waste of this famous victory if we didn't go and reach the final and lift the trophy."
Diarra, signed from Arsenal for £5.5million in January, added: "Last year I got to the final with Chelsea and sat on the bench but now I want to actually get on the pitch with Portsmouth and win the Cup."