I assume you're talking about Michael Piller. You want to know the funny thing, though? He was the guy who turned The Next Generation around after its weak early seasons, and was the writer of the highly acclaimed The Best of Both Worlds. So it's a bit funny that his arrival so greatly improved The Next Generation, but Deep Space Nine improved after his departure.
Though to be honest, I'm not sure if it was Michael Piller's departure that made the difference in DS9 as much as Ira Steven Behr becoming the guy in charge of the series...maybe a bit of both.
EDIT: Whoops! I meant to add this into my previous message via edit, but goofed up and made two separate messages.