Why did Japan use Yu Darvish at the 9th inning of Japans win over USA sunday?

I just thought this was wierd, 5 run lead in the 9th I would not use my star pitcher which could start on monday against korea. I also thought it was a bit wierd how they took Dice-K off after just 4 innings of good work. Do you think they will use darvish against korea as a relief in case Iwakuma fails?