Chrysler appears to be taken by surprise over Ram-brand President and CEO Fred Diaz’s sudden departure for a senior-management post at Nissan.

“So I guess it’s true,” Rick Deneau, Chrysler’s top U.S. spokesman, tells WardsAuto.

Other insiders contacted also say they had no idea Diaz was looking to leave the Auburn Hills, MI-based auto maker.

Diaz will fill the newly created position of divisional vice president-Nissan Sales & Marketing for Nissan U.S.

Chrysler officials say it is unclear when a replacement will be named for Diaz, who also served as president and CEO-Chrysler de Mexico.

Diaz’s departure comes at a time of increased momentum for Chrysler, where Ram has performed especially well since the executive took the reins of the new brand in 2009.

Under Diaz’s guidance, the 1500 pickup underwent a major redesign and garnered several industry awards, including North American Truck of the Year, becoming a best-selling vehicle for Chrysler as a result.

Diaz also steered the introduction of the ProMaster van, arriving at dealerships later this year. Additionally, he heavily courted Hispanic-American customers, which he foresaw as a growing pickup-buying demographic in the U.S.

“Fred Diaz is one of the most experienced, results-driven sales and marketing executives in our industry, and we are privileged to welcome him to Nissan,” says Jose Muñoz, Nissan senior vice president-sales and marketing for Nissan Americas.

Diaz’s expertise in successfully selling the Ram was no doubt a key reason the Japanese auto maker wanted him to join its U.S. team.

Nissan is planning to launch the next-generation Titan fullsize pickup truck next year.

The current Titan debuted in 2003 as an ’04 model and quickly fell behind Nissan’s modest sales expectations of 100,000 units annually in the U.S.

There was a question whether the auto maker even would take another crack at the fullsize-pickup segment, given that the U.S. Big Three dominate the sector.

Diaz also arrives at Nissan at a time when, despite a range of new models such as the Altima midsize and Sentra compact sedans, sales have been down.

In four of the last seven months, Nissan U.S. deliveries have fallen below year-ago.

Thanks to a strong March, when incentivizing helped push Altima sales ahead of Toyota’s Camry, the Nissan brand eked out a 4.1% increase, WardsAuto data shows.

Ram sales were up 30.1% in March.

– with Aaron Foley

cschweinsberg@wardsauto.com