The Spanish midfielder is under contract at the club until 2019, but insists he is no nearer to signing a new deal.
But while he is at the club for another year, he has revealed he is unsure of his future beyond that - with Barcelona having been keen on him last year.
He told Mundo Deportivo: "I will definitely see out the year I have left but I don't know if I will sign a new one.
"We have started talking, but nothing has been decided yet."
He was an early favourite of Mourinho, but has seen his playing time dwindle since the arrival of Nemanja Matic from Chelsea last summer.
A host of clubs were looking at the combative Herrera in this season's transfer windows, yet the fans' favourite made it clear he wanted to stay.
Lack of game time also means he could potentially fail to make Spain's World Cup squad for Russia this summer - a blow to a man who has cited playing for his country as the highest honour.
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