He's 20 years old and up until last year was still playing in Croatia, what do you have expected him to do? Players like Modric, Mandzukic and Brozovic were still playing in Croatia as 23/24 year olds.Nothing he has done in his career so far suggests that he’s remotely close to Sigurdsson ability wise.
He scored in the Europa League as a 16 year old, became Hajduk's captain as an 18 year old, made his full debut for Croatia (who have one of the best midfields in the world) as an 18 year old. Played less than 600 minutes of premier league football, most of them still as a teenager, scored 2 for us in Europa league games and is now CSKA's key player and direct replacement for a fella they sold for 30 million in the summer.