Personally I think folks underestimate just how hard it is to play the centre forward role at a top club in the PL, it's probably the most difficult role on the pitch. In most games you have guys like Auba, Jesus, Werner, Abraham really struggle to get involved. Guys like Watkins and Calvert-Lewin thrive in counter attacking sides, they'd get eaten alive for teams that come up against deep, compact defences.
Auba might struggle to get involved, but he'll score goals up there. Just stick with him until a genuinely world class talent presents themselves, or just move Martinelli up top too. These mid table options will let you down.