The 19-year-old has been linked with a move Manchester United, after demanding Sir Alex Ferguson's attention with a superb goalscoring display in the Basque side's 3-2 win at Old Trafford in the Europa League.
Juventus have also been linked with the Spanish youngster, who made his international debut earlier this season. However, his agent Jesus Medina has warned that the player will not leave for a penny less than his buyout clause.
Medina told Calciomercato.com: "Athletic do not want to sell. Whoever wants him must pay his buyout clause at €36m [£29.5m]."
Muniain has scored five Europa League goals in 13 appearances this season.