Or at least delay the march to a more indirect route. As you know, he'll(Napoleon) have to relocate his supply magazines, depots, and hospitals in his line of communication and realocate several regiments and even brigades to protect them. This could delay him a month or more as he moves his route to Silesia as I anticipate. At best even force him to wait until the Spring to complete his plans.
This will give the Grand Duke time to prepare. He cannot depend on the Russians to deliver their assistance in a timely manner, as they may very well have their own designs. But you can depend on the Bunnies.