He's scored 8 this season (in 20 games). Last season was poor but he had a series of (minor) injuries at the start of it, plus Real Sociedad were piss poor most of the season, changed managers midway through. No one did particularly well, though he was still second highest scorer behind Mikel Oyarzabal who scored 13.7 goals this season, 11 last season 2017/18 was the best of his career with 20
Willian José - Wikipedia
en.wikipedia.org
Arguably his best days are behind him, 28 moving to a new country where he needs to be scoring straight away, a short term fix for a long term problem.
Not sure why you'd think "his best days are behind him", presumably that opinion is based solely on his Wikipedia page in English. His goals per game average for La Real is a respectable 0.43, Ziatek's over the same period is 4