Well Matthews and Marner have always wilted when the pressure ramps up, space disappears and teams play them tough. They're players that seem to care more about the name on the back of the jersey than the crest on the front.
It's no surprise that Toronto has trouble finding success when this is a duo that seems to run the organization as well. They are stapled to the hip, demanding that they play with one another at all times and Keefe apparently isn't allowed to hold them accountable as we've seen in the past when any criticism is immediately walked back. Tavares is an empty suit and not a leader who demands any sort of accountability. Nylander, actually, is the only one of the quartet who actually seems able to adapt to playoff style hockey.
As long as Matthews gets his 70 goals and Marner gets his 100 points and continues to cosplay referee, all is good in Leaf Land.