You seem to be under the impression that the WK was away often. It was not so. As far as we know, he permanently stayed in Minas Morgul from TA 2002 till TA 3018. And then Sauron sent him (and the rest of the nazgul) for the Ring - thus the WK was absent from June till November 3018 - only 5 months in a thousand years.
Sauron may have appointed someone as head of the army to replace the WK for these 5 months, but surely before the actual war new arrangements were made.
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