Bearing in mind also that the Prussians re-organizing themselves, rallying and marching to Wavre was not what the French were expecting, it seemed likely after Ligny that the Prussians would surely retreat East, and the early reports indicated this, and any other army commanders but Blucher and his staff probably would've done. But it was the skill and leadership of the Prussian commanders that kept their army together and pushed these troops on and on to Wavre then Waterloo. The Prussian army conducted unprecedented accomplishments in the Waterloo campaign, they suffered through skirmishing with the French on the 14th and 15th, then suffered a demoralizing setback at Ligny, but it still kept going, marching through the night to Wavre then the early morning of the 18th through boggy and poor roads to reach Waterloo.
Grouchy would never have even guessed that the Prussians were already preparing to march on Waterloo, even after his delays.