Los Leones have rejected a reported £25m bid for the Spaniard on Thursday.
"Ander is getting on very well at Athletic, very content," Pedro Herrera told Mundo Deportivo. "He's fought hard to come to this club and it's very difficult for him to leave.
"I think it's Manchester United's decision, since they have not found a replacement for Cesc Fabregas. They have decided to opt for a creative player and have thought about Ander."
The former Spanish Under-21 international joined Bilbao from Real Zaragoza two years ago.