United are keen for the player to stay but are mindful that foreign clubs would be free to talk to the Spain worldwide from January.
But United are ready to protect their investment just in case by triggering the one-year extension in the four-year contract Herrera signed with the club when he arrived from Athletic Bilbao in a £30million deal in the summer of 2014.
But United had hoped to conclude a deal by now and if there is not a significant breakthrough shortly the club are likely to activate the option in Herrera's contract given that he would otherwise be a free agent next summer and free to talk to foreign clubs in January.
United triggered a 12-month option on Marouane Fellaini's contract a year ago to ward off interest from China.
Pogba posted an Instagram video on Thursday stepping up his rehabilitation in Miami but Mourinho has no timescale over the former Juventus player's comeback but was more confident on Eric Bailly's return which could see the defender return for next Saturday's meeting against Tottenham, live on Sky Sports Premier League.
"If you ask if he can play next week against Tottenham then I don't know".
Eric Bailly and Marouane Fellaini are also sidelined with injures and will miss the trip to Huddersfield.
Despite being voted United's Player of the Year last season questions had been made over the midfielder's future prospects at the club after he lost out on his place to new arrival Nemanja Matic.