Eve of what battle?
Napoleon did not believe there would be any battles on the 16th - but if there was to be one, it would have been on the right wing, and extremely minor.
Time will tell all - and this analysis comes directly from the correspondence, mostly Napoleon's.
The mere fact that you ask the question that way demonstrates how flawed the histories of this campaign are, and how much as been ignored in lieu of evaluating the actors based on what actually happened and where forces actually were, rather than only from what they knew (or thought, or had been told) at each moment in time.
Regardless, while Napoleon expected Ney to be simply minding the left and hanging out until he could direct the drive on Brussels, it was presumptive, and when things changed, it hurt his cause... but on the flip side, if Ney doesn't show up, and an Anglo-Dutch army started driving against the flank from Mons, does Napoleon want two Lt. Generals who are suddenly Corps commanders to coordinate a combined defense?
Napoleon didn't order Ney to the left wing to take significant offensive operations - he ordered him there to push a few battalions out of the way and sit tight.