I don't think we should worry about his decline, those numbers are probably because for almost a year now he knows he is leaving porto, and there is no way, no matter how big of a professional you are, you don't engage an autopilot. Also, he doesn't strike me as a striker that relies on speed or rapid movements, things that get you in a rapid decline seat, instead he is a poacher type with solid passing abilities that can probably play well into his late 30s. Also, porto was kinda shit in the league this year as well.
The only question mark I have around him is how much of his quality in portuguese league which in undeniable imo could be translatable in seria a. Cos, all sorts of bums scored 30 plus goals in that league...