I think the benchmark and the ultimate goal for Coutinho has to be Zidane – the best player in recent history who combined traits of #8 and #10 (you could say the same about Iniesta, but his game is noticeably less direct than that of Zidane or of Coutinho). Zidane has scored on average 1 goal in 5 games in his career. He has scored 10 goals in the league only once at age 20 at Bordeaux, he reached 9 league goals twice with Real Madrid and his highest season total at Juventus was 7. He had roughly the same number of assists as goals throughout his career (unlike Iniesta who has more than twice as many assists as goals).
Zidane age 20 – 35 league games, 10 goals 6 assists (Bordeaux)
Coutinho age 20 – 23 league games, 4 goals 5 assists (Inter and Liverpool)
Zidane age 21 – 34 league games, 6 goals 8 assists (Bordeaux)
Coutinho age 21 – 33 league games, 5 goals 7 assists (Liverpool)
Zidane age 22 – 37 league games, 6 goals 4 assists (Bordeaux)
Coutinho age 22 (season still in progress) – 23 league games, 4 goals 4 assists (Liverpool)
Stats taken from here and here.
Zidane age 20 – 35 league games, 10 goals 6 assists (Bordeaux)
Coutinho age 20 – 23 league games, 4 goals 5 assists (Inter and Liverpool)
Zidane age 21 – 34 league games, 6 goals 8 assists (Bordeaux)
Coutinho age 21 – 33 league games, 5 goals 7 assists (Liverpool)
Zidane age 22 – 37 league games, 6 goals 4 assists (Bordeaux)
Coutinho age 22 (season still in progress) – 23 league games, 4 goals 4 assists (Liverpool)
Stats taken from here and here.