Mate. very very few strikers score 20 goals plus in a premier season. Think Rooney only managed it twice in about 15 attempts. Lukaku has only done it once in 6 attempts. The ones that have managed it in the last 10 years have all been top draw strikers. Rooney had a lot of penalties in that tally last season and, at the end of the day, still only ended up with 10.
If Tosun can get north of 15 goals next season I think that would be a great return. Last season 15 goals would put him 7th, on a par with Firmino and behind Salah, Kane, Aguero, Vardey, Sterling and Lukaku. With the exception of Vardey, all of those players would probably cost £100m in the current market. To expect more from a £20/30m striker like Tosun is unfair.
We sometimes talk about 20 goal strikers like they were sweets.