With the current cap situation on the Maple Leafs, after the Zaitsev trade, they have enough room to sign Mitch Marner, have Cody Ceci accept his qualifying offer of $4.3 million and add another minimum salary player at best.
There is no room there for an upgrade on defence because the Leafs were unable, or unwilling, to move out Zaitsev without taking back a nearly equivalent salary for this year. The reality is, the cost might have been too high to have avoided this outcome, and they need someone to play defence..
That leaves the Leafs with one option to actually add a player of note that doesn’t involve a trade of one of their younger stars: moving Nazem Kadri.