He didn't play with any pace and wasn't a consistent offensive threat throughout the series. For large chucks of the time he was hemmed in his own zone and not moving his feet through the neutral zone. His point totals were very flattering based on the impact he had on a game by game basis.
If the Leafs are going to win they need Marner to be creating multiple scoring chances a period. Playoffs swing so much momentum shift by shift, he needs to create that momentum. Matthews to a large degree was doing this by himself when they were on the ice together.