Because of the massive contracts that the "Core 4" as well as Morgan Rielly are signed to, the Leafs are going to be up against the salary cap unless they can find a way to move out a contract without also leaving a gaping hole in their roster. This is just one reason why a rental seems to be Dubas' preference.
Shilton has speculated that the Leafs may be interested in going after a player who plays a similar style to Jake Muzzin. Muzzin got hurt in both of the Leafs' last two playoff runs and their defense took a nosedive afterwards. Despite his struggles this season, his presence is missed when he's out of the lineup.
With less than two weeks until the trade deadline, the Leafs haven't made a huge splash yet - and with Dubas already confirming that the Leafs have one move remaining and that they'd prefer to make it a defenseman, it's just a matter of time until a move is made now.
Of course, Dubas already made one move, bringing in a physical, right-shot, defensive defenseman in Ilya Lyubushkin, so it's not as though he has stood pat. Leafs Nation is hoping that the last move for Dubas is a big one - but even if it isn't a bigger deal, I'll settle for the right deal.