The center wasn't Tavares' man at that moment. Your man changes depending on the situation. Tavares had been in on the forecheck, and was attacking the puck carrier on the way back as the highest forward. Nylander also comes up to pressure the puck carrier, and then realizes Tavares has it, does a little twirl at center ice, and then glides at the blueline and reacts late as Cirelli skates past him.
The faceoff loss had nothing to do with the goal. Go back and watch. Nylander pretty much doesn't move or do anything for the full 10 seconds after the faceoff is lost. He just kind of stands in the middle of the zone around nobody, then drifts too high giving the Tampa player a wide open lane to cut into the middle, and then doesn't even bother to poke backwards at the puck carrier to disrupt the shot when he realizes. Love Nylander, but that's just not good enough, especially in the playoffs.