Time will tell, but I don't see Cohen as a true handcuff because of his non-prototypical size at the position. This is not to say short guys can't be stars in the NFL (MJD anyone?) or that lighter backs can't stand up to the rigors of the role (hello, Warrick Dunn). But the combination of Cohen's size/weight (5'6, 179) puts him in a class of player whose probability of standing up the rigors of a 3-down back in the NFL is less favorable.
The comparison to backs like Sproles and McCluster is apt -- Cohen's skill set and talent can make him an asset on any down, using his combination of speed and elusiveness (some owing to his size) to pull off creative plays as a back or receiver. But I don't think Cohen is a guy who has the power/type of game to be able to consistently run through the tackles or have consistent success running north/south.
As such, I think the way Cohen can eat into Howard's carries is as others have outlined -- with a need for a spark as teams simply stack the box against what looks like an anemic pass game for CHI this year, Cohen should be given opportunities to move the chains.
But though Howard may lose some touches to Cohen, I see the amount of volume that's coming Howard's way not changing significantly, and I don't see that Cohen would be the sole answer for CHI's backfield if Howard goes down with injury.