I'm guessing here, but there's a real chance United will have Rooney and RVP back for the final, if United beat Sunderland. With those two playing, they can beat almost any team.
To wtite them off with a full team is madness.
As for new players. It's all good and well signing them, if United have the money, but it will take time for them to adapt to the a Premier League. Not to mention having a Manager who can make the most of those new players.
And don't forget, the teams finishing abovd United will strengthen as well.