Hallertau Hersbrucker isn't a Hallertau Hop, its Hersbruck grown in the Hallertau region. Although Hersbruck is a very nice hop it wouldn't be an ideal substitute in this case.
German hop names have a system, region, breed, sub-breed (sometimes), so more properly it would be Hallertau Hallertauer Mittlefruh.
The traditional version of Hersbrucker is Hersbruck Hersbrucker, tells you its Hersbrucker grown in its home region. Because hops have been grown in the Hallertau region for so long (its likely that's where they evolved) there are lots of pests/diseases that prey on Hallertauer variety it is now often grown in other regions where there are less pests. Other varieties are now grown in the Hallertau and there is a lot of work going on to develop higher alpha more resistant sub-breeds of Hallertauer, that retain the properties that made Mittlefruh one of the most desirable hops in the world..
Mark