Really stupid time to sack the manager when you look at those fixtures.
They should have either done it after the Man U game or wait until after the Arsenal match. Surely any "new manager" syndrome (if there is one; it seems as though the players were giving Hart 100% already) will be gone pretty quickly even if they pull a result or two out of the bag as there's no way they will string an undefeated run together with those matches.
I feel sorry for Paul Hart. If I was him I would leave Pompey completely. He surely merits a manager's role at a good Championship club.