Herrera was again linked with United last month.
He told El Confidencial: "I made it clear at the time.
"I am grateful to United for their interest and to Athletic for considering me to be more valuable than money.
"Athletic have made it clear that they have no intention of selling.
"We are a solid club with a good economic standing. We are privileged.
"We are stable financially, well respected, have a new stadium and a great team - no Athletic players want to leave."
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