Rosas was hired Aug. 1 to fill a position that had been vacant since former coach and GM Don Nelson left the franchise eight years ago. Most of those duties fell to Nelson's son, Donnie, the president of basketball operations.
Dallas owner Mark Cuban asked the younger Nelson to find an addition for the front office with stronger organizational skills, and Nelson came back with Rosas, who had been an assistant to general manager Daryl Morey in Houston.
The 35-year-old Rosas, who reported to Nelson, said in a statement issued by the team that the job "was not the best fit for me at this point in my career."
"The decision was made solely by me, as I firmly believe this change is in my best interest," Rosas said. "Mark Cuban asked me to reconsider my decision, but graciously accepted my decision and we part as friends."
Rosas, a native of Colombia, spent nine years with the Rockets, rising from an intern to vice president of basketball operations. He played a role in shaping the roster for a Houston franchise that beat out the Mavericks, Los Angeles Lakers and Atlanta in the pursuit of free agent center Dwight Howard.
Rosas also spent time as general manager of the Rio Grande Vipers, the Rockets' developmental team. A decade ago, he was a graduate assistant with the University of Houston men's team. He was also a basketball coordinator for the Venezuelan national team at the 2002 world championships.
"I was disappointed when Gersson told me of his decision, but I understand and all of us wish him the best," Cuban said.
Rosas didn't return a call from The Associated Press seeking further comment, and Cuban didn't respond to an email.
The Mavericks open the season Wednesday night at home against Atlanta.
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