Nylander and Tavares aren't a natural fit.
When Matthews was out and Marner was playing with Tavares it was a nice refresher on what Tavares needs and hasn't gotten on the second line. Marner was the principle puck carrier and did a nice job running routes that bought Tavares time to catch up on the play, shrink the area of the attack down and set up little area plays closer to the net. Basically kill open ice.
On the other hand, Nylander wants to live off the rush/counter rush and had numerous breakaways in the playoffs to demonstrate this. Kerfoot can play that game but Tavares doesn't do well if it's a back and forth track meet.