Kelty was crucial to the creation of the first Gigafactory in Nevada. A former Panasonic employee, he was the chief negotiatior for the Gigafactory partnership. Tesla wanted Panasonic's help keeping up with demand (particularly when the Model 3 arrived), and Kelty got it. He also regularly made "billion dollar" deals with other suppliers, and led both quality control and recycling efforts. His departure leaves big shoes to fill, and there's no guarantee that the successor will clinch future agreements with similar skill.
Is Tesla going to reel from the loss, though? Not necessarily. You could argue that Kelty overcame the biggest hurdles Tesla will have to face for a while: he got established battery companies to take a chance on an EV startup whose very survival was far from guaranteed. Tesla isn't out of the woods yet, but the focus now is more on expanding those deals as the firm widens its car lineup and opens more factories. Whoever replaces Kelty will still need to make hard decisions, but they won't have as many make-or-break situations on their hands.