The Hurricanes selected Johnson, a 19-year-old Michigan sophomore, with the No. 3 overall pick in last year's draft, but twice in the past year he turned down offers from the Hurricanes to turn professional. That prompted Carolina general manager Jim Rutherford to entertain trade offers.
"This was clearly the best offer and clearly the best for the Hurricanes," Rutherford said. "We've kind of gone back and forth -- do we wait for the development of Jack Johnson, and when does that take place, or do we take a player now that's a good young defenseman who's going to be good for a long time? As camp has wound down, that was the conclusion we all came to, that now was the time to do it."
Johnson said the trade would not affect his timetable for turning pro and that he is concentrating on his upcoming season with the Wolverines.
"I'm here in Michigan, and that's where my focus is," Johnson said. "I'm excited about my new team, but my life isn't going to change because of it."
Kings general manager Dean Lombardi said he supported Johnson staying in school.
"We were prepared to be patient when we made this deal," Lombardi said. "It'll be good for him to spend another year there."
Rutherford said he had decided last week not to trade Johnson, but then talks with the Kings picked up.
"We just felt that we're getting a very good NHL defenseman in Tim Gleason," Rutherford said. "He's still a young guy, he still has an upside, and both of those players will come in and be with us now, instead of waiting for the development of Jack Johnson, who in my opinion is going to be a very, very good, if not great, NHL player, but probably not for another three to four years.
"Everybody has decisions to make. They wanted to spend another year in school. I think that, in his best interests, for his development, that he should be playing pro now. He's ready to do that. The sooner he starts playing pro, the sooner he starts to develop into a real good NHL player," he said.
Rutherford said he wasn't sure whether the trade would prompt Johnson to turn professional, saying, "We can't be concerned about that. We got two players we feel made our team a lot stronger."
Lombardi acknowledged that Belanger and Gleason could have helped the Kings immediately, but he wanted to gamble on Johnson.
"He potentially fills a position that's hard to find. These type players aren't often available at a young age. We felt we had to be fairly aggressive," he said. "The kid is a competitor. He's going to be every bit as good as he can be."
Belanger, 29, is a center who had 17 goals and 20 assists in 65 games last season with the Kings. In five seasons in Los Angeles, he had 63 goals and 87 assists in 323 career games.
Gleason is entering his third NHL season. The 23-year-old defenseman had two goals, 26 assists and 98 penalty minutes in 125 games with the Kings.
The Hurricanes included Tverdovsky in the deal because the Kings wanted an NHL-ready defenseman to replace Gleason, and Carolina was seeking financial flexibility in the future, Rutherford said.
Tverdovsky, 30, had three goals, 20 assists and 37 penalty minutes last season, his only one with the Hurricanes.
The move came hours before the Hurricanes' final home game of the preseason against the Columbus Blue Jackets. Rutherford said Belanger and Gleason would join the team Saturday and play in Sunday's preseason finale at Washington.