There is not a single living, breathing Yankees fan on this planet who would say any given series against the Red Sox isn't the most important matchup on the schedule at any point in time — in most instances, of course. Typically, these teams are jockeying for playoff spots/positioning, and the fact they are hated divion rivals just adds more juice to the contests.
But don't tell that to manager Aaron Boone! We know his philosophy. You simply can't get too high or too low with your emotions during the baseball season. You must maintain equilibrium until the playoffs or else you've lost the plot.