As said, this card has the opposite effect of overdrive algebraically, but of course, attack is more important than a certain creature health, and therefore this card is underpowered compared to overdrive and other cards.
However, how about having a kind of opposite effect of rage potion, so that it could be used with cards like rage potion?
Something like:
Unupgraded: Cost 3; Replace the target creature skill with Slow Down: gain -3 / +5 per turn.
Upgraded: Cost 4; Replace the target creature skill with Decelerate: gain -2 / +5 per turn.
EDIT: Looking back on it, looks a bit powerful;; but anyways, any thoughts?