Chrysler CEO Sergio Marchionne and new Ram Truck boss Reid Bigland.

After unexpectedly losing one of its top brand bosses to Nissan, the Chrysler Group is shaking up its executive lineup, putting new managers in charge at its Ram, Dodge and Fiat brands.

The quick switch following the loss of Ram chief Fred Diaz, appears designed to maintain momentum at a maker that has posted three years of month-over-month sales gains. But while Chrysler might be showing off the “depth of our talent pool,” the shake-up also reveals how a handful of top executives are each being asked to handle multiple assignments in an unusual lean corporate structure.

“These appointments are part of the Chrysler Group’s strategy to continuously reinforce its leadership ranks as we continue on our path to achieve our business targets,” declared Sergio Marchionne, chairman and chief executive officer of Chrysler Group LLC, in a statement issued Monday morning.

The changes come just as Chrysler is ramping up production of the new Ram 1500 pickup – which was named North American Truck of the Year in January — launching its new commercial vehicle division and preparing for the introduction of a new series of Fiat and Dodge products between the fourth quarter of 2013 and the first quarter of 2014.

Among the most immediate changes, Reid Bigland takes over as President and CEO of the Ram Truck Brand. He replaces Diaz, a 24-year Chrysler veteran who made a surprise move to Nissan last week, taking over as the U.S. sales and marketing chief for the Nissan brand.

(Diaz Defection Good News for Nissan, Bad News for Chrysler. Click Herefor that story.)

Like Diaz prior to his departure, Bigland is among a handful of trusted lieutenants in Marchionne’s inner circle – and like the rest of that cadre, he wears a number of different management hats. Bigland will continues as head of U.S. Sales for Chrysler, and as President and CEO of Chrysler Canada, according to the announcement. Bigland came to Chrysler in July 2006 from Freightliner Custom Chassis Corporation, a South Carolina-based company.

Another key change moves Timothy Kuniskis to Dodge, where he replaces Bigland as president and chief executive. Kuniskis most recently served as Head of the Fiat Brand for North America, helping build momentum for the Italian marque after an initially disastrous launch in 2010.

Kuniskis joined Chrysler in 1992 and has held a series of increasingly responsible positions in the company’s business center operations and marketing organization.

Taking over for Kuniskis at Fiat is Jason Stoicevich, a 10-year Chrysler veteran who retains the position as director of the California Business Center. Stoicevich has held a variety of assignments in the company’s business center operations, he said.

Meanwhile, Bruno Cattori has been appointed President and CEO of Chrysler de México. Cattori most recently served as Director for Retail Sales and Fleet for all Chrysler Group brands in Mexico, a position which he retrains. Prior to rejoining Chrysler Group in early 2012, he served as President and CEO of Mercedes-Benz Mexico. Cattori began his career at Chrysler in 1988.

The various new appointments serve as “a further confirmation of the depth of our talent pool and of the exceptional opportunities that are available to our people as part of their career development,” Marchionne said.

But analysts point to the fact that senior executives at Chrysler are now required to serve multiple duties and question whether the maker might be pushing its management team a little too hard. Several senior sources complained to TheDetroitBureau.com that they are working constant 7-day schedules with virtually no time available for rest or families.

That appears to have been at least one of the factors behind the decision by Diaz to take the Nissan job.