Sauron and Saruman?

[the real findorfin]

As I seem to remember (and I seem to recall this might be the Mouth of Sauron's own thoughts and not Sauron himself), Sauron's plan was to install the Mouth of Sauron as lord of Eriador from Isengard. I wonder if this was a change in his plan afer he learnt that Saruman had fallen or was it planned from the start, i.e. use Saruman until the war is finished then discard him and install a more trustworthy captain? If this was not the case then I wonder where else he would have installed The Mouth, if Saruman was to hold Isengard still? Just a thought I had.