United have already had a £21million bid for the 24-year-old rejected by the Spanish side.
Bilbao want United to meet Herrera’s £25.5million buyout clause.
Bilbao president Jose Urrutia said: “For a player to leave he needs to state his intentions to leave and (the club) pay the buyout clause.”
Reports claim Herrera is keen on a move to United before the transfer window closes.
