Monreal left Arsenal for La Real in August.
And he revealed: "Yes there were chances of signing for Athletic. I'm not going to tell you that I was very, very close, but there were possibilities.
"The only reason I have not finished in Athletic was because the three parties have to agree and, for one reason or another, that agreement has never been reached."
Monreal also praised coach Imanol Alguacil: "I am not surprised by the courage proposed by the mister. We want to attack always. We are going to death with him."
And on Manchester City target Mikel Oyarzabal, he added: "He is one of the best in the team. It is a luxury to have a player like him."