I think the new-ball should no longer be given to him though, its important that Australia make a good start with the new ball, because its the new ball that has been doing most of the damage in this series so far, and Mitch unfortunately has been spraying it all over the place.
I think he is trying too hard to swing the new-ball which is one of things that's causing him these problems, as stephen pointed out, he has the ability to be extremely hostile with the ball, and he should concentrate on rattling and intimidating the batsmen, if the balls swings or reverses for him then he should just take it as a bonus, but he shouldn't really lose sleep over it, as he can still be very dangerous, even without swinging the ball.