Exact reasons why Keller is leaving AMD aren't revealed but the company claims his departure will have no impact on the development of Zen. Keller's role as Zen team leader will be taken over by Mark Papermaster, AMD's Chief Technology Officer.
Attempting to allay any fears over the impact of Keller's departure, the company adds that "Jim helped establish a strong leadership team that is well positioned for success as we enter the completion phase of the “Zen” core and associated system IP and SoCs."Source: Hexus
"Jim’s departure is not expected to impact our public product or technology roadmaps, and we remain on track for “Zen” sampling in 2016 with first full year of revenue in 2017."