If you want a fair comparison...
Diaz was decent as a 9, probably the best 9 amongst all forwards, as proven by the number of games he started as a 9.
However his numbers dropped drastically when he played on the LW when Gakpo was out. We missed Gakpo's output because Diaz just couldn't produce anywhere near the same numbers as Gakpo was producing on the left. We were duly knocked out of the CL and lost the league cup during this period.
It is no coincidence. Diaz numbers on the left for 3 seasons has always been average and cannot match up to the numbers Gakpo had produced this season at all.
Indeed. Diaz scored 8 of his goals as a CF, so in terms of output from the LW its not even close stat wise.