Kadri is a good #2 centre, and a great #3 centre.
The Leafs obvious weakness is on D.... so at first glance, it's an opportunity to trade from an area of strength. However, even if you get an "equivalent value" decent #2/great #3 defenceman... the Leafs are still going to be unspectacular on D.
The way the Leafs have (and are going to have to) get by without having a spectacular D, is by forcing teams onto their heels. A potent, dangerous, and fast counterattack -- to relieve pressure off that suspect D. Having a great counterattack starts with the middle of the ice with your centre -- this is how babcock succeeded in Detroit all those years despite having defencemen that weren't always the best on paper.
With Kadri/Matthews/Tavares, there's really only one team who stands out as having a better group of 3 centres than the Leaf. Without Kadri, they're an extremely top-heavy group who are one injury away from falling apart, especially considering that the Leafs really don't have an NHL-qualified 4th line C.