Everton boss Marco Silva says the No10 is the most important player of his system.
Silva has employed his trademark 4-2-3-1 formation for the majority of his reign with Everton.
"My ideas and approaches have stayed largely the same," he told Sport TV in Portugal.
"You look at the role of the number 10 in my 4-2-3-1 at Estoril [Evandro Goebel], then Joao Mario at Sporting, then [Kostas] Fortounis at Olympiakos who had his best ever season and now the same thing happens with Sigurdsson, having his best goalscoring Premier League season. Players start scoring at a rate like they never did in their careers."