The English born Ghanaian opened the scoring in the 26 minute after taking advantage of a mistake by the Hammers.
West Ham's Xande Silva was left unmarked inside the box and he made no mistake in finding the back of the net with a header from close range.
West Ham then went on to enjoy a spell of possession and work a handful of half-chances, but after playing a long ball over the Hammers defence, Nketiah found himself through on goal and he found the bottom corner.
Joe Willock then tried his luck with a long-range effort, but to Arsenal's frustration, the Hammers would reduce the deficit once more through Reece Oxford, who bundled the ball in from six yards out.
Ten minutes later Pleguezuelo got the advantage with a looping header from distance for Arsenal.
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