The Spanish midfielder still has a little over 12 months left to run on his current terms, which the Red Devils are reportedly keen to extend.
United took up the option to add an additional year to Herrera's deal to take him through until the summer of 2019, but the 28-year-old is unsure if his time at Old Trafford will continue beyond that point.
"I will definitely see out the year I have left but I don't know if I will sign a new one," he told Mundo Deportivo. "We have started talking, but nothing has been decided yet."
Herrera has started just nine Premier League games for United this season, compared to 27 throughout the 2016-17 campaign.