The Red Devils' player of the year last season had expected to hear by now about a four-year extension which had initially been mooted by the club's hierarchy.
Herrera is out of contract next summer and United will be taking up their option to extend that by a further 12 months to avoid the risk of the Spain midfielder walking away for free.
The 28-year-old's representatives, however, are said to be a "little bemused" the club are still dragging their heels over putting their client's long-term future on a much more established footing.
Herrera is believed to earn around £130,000 per week, with one theory being that United do not want to start paying out bigger wages until they have to, with reports the player is now looking for around £200,000 per week.
There is no indication whatsoever that Herrera wants to jump ship, or that United will not get the deal done on their timeline, although the delay is a distraction, with the former Athletic Bilbao man now set for an extended run in the team due to the knee injury suffered by Marouane Fellaini on international duty.