Boston General Manager Danny Ainge won the award, announced by the Sporting News, which polled general managers and other executives from all 30 NBA teams. Ainge received 18 of the 47 votes, Kupchak had 14 votes, and New Orleans General Manager Jeff Bower was third with 12 votes.
Kupchak received voters' attention in part because of the February trade that landed Pau Gasol and revived the Lakers' season. He also stood pat when Kobe Bryant wanted to be traded during the off-season and resisted numerous offers for Andrew Bynum.
The Lakers finished with the best regular-season record in the Western Conference.
"I don't see why he shouldn't finish first," forward Lamar Odom said. "Thinking of some of the guys that he picked, people kind of wondered about that. To understand what Bynum was going to do and what he was going to bring, he's definitely deserving of it. And then, to be able to pull off that Gasol trade was amazing, especially in the middle of the year like that."
And, Odom added, "He did a great job by keeping this team together."
Ainge orchestrated the off-season trades that brought perennial All-Stars Kevin Garnett and Ray Allen to the Celtics. Ainge, who is also the team's executive director of basketball operations, is the first Celtic to receive the award since its inception in 1973.
Kupchak said Ainge was the rightful winner.
"The guy that deserved it got it, and I'm happy for him," Kupchak said. "What he did there was just incredible."
Ainge was presented the award before Wednesday's Cleveland-Boston game.
The Lakers aren't the only top-seeded team having trouble on the road.
Boston is 0-5 away from home in the playoffs, making the Lakers' 2-2 road record look substantially better.
"The East is turning into quite a contest for the Celtics to get through," Lakers Coach Phil Jackson said. "They had their way early in the season and I think the rest of their conference has caught up a lot with what they do.
"What happens is a lot of teams will copy what works with defending other players. They've been able to consistently hold one [of their three big scorers], maybe even two of them, sometimes three of them, down below their averages during road games. That's been a big difference."