By Gary Washburn and Julian Benbow, Globe Staff

DENVER -- The Celtics, according to an NBA source, have traded popular workhorse center Kendrick Perkins to the Oklahoma City Thunder along with guard Nate Robinson for small forward Jeff Green and center Nenad Krstic.

The Globe first reported that Robinson would be traded while Yahoo! Sports reported the deal with Oklahoma City. The Globe also learned the Celtics will receive the Clippers' 2012 first-round pick in the deal.

Perkins, a Celtic since entering the draft out of high school, was a fan-favorite in Boston because of his tough-minded style and exponential improvement over the years. But he was a free agent this summer and the Celtics sensed issues with re-signing him for the long term. Perkins is from Texas and will get a chance to play closer to his home.

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A look at the trade

• Discuss the trade
• Player stats for Jeff Green
• Player stats for Nenad Krstic
• Reports: Marquis Daniels traded for cash
• Reports: Celtics trade Erden, Harangody for draft pick

Perkins, who is expected to miss the next few games with a sore left knee, attended the team's shootaround at Pepsi Center and was joking with teammates as they entered the arena. While Perkins wanted to stay in Boston, he also wanted to be compensated like many of the league's top centers and was resigned to the fact that it might not be in Boston.

"With Perk, everybody thinks he's mean because of how he looks," said Robinson. "But he's a soft giant. He has a good heart ...He's taking it pretty hard because he's been here eight years. He was real emotional, crying. I feel bad for him.

"I told him he's not alone. We're going together. It's not like you'll be by yourself."

Green was fifth overall pick in the 2007 draft by Boston and was traded to the Seatte SuperSonics in the Ray Allen deal on draft night and was one of the young pieces for the Oklahoma City Thunder. But with the emergence of Kevin Durant and Russell Westbrook, the Thunder did not sign Green to a contract this past summer, making him a restricted free agent when the season ends.

He is the small forward the Celtics had been seeking and will give coach Doc Rivers many lineup options because he can also play power forward. He was averaging 15.2 points and 5.6 rebounds for Oklahoma City.

Robinson came to the Celtics during last year's trade deadline and was included in this deal because his salary matched and the return of Delonte West would take some of his minutes.

Robinson said goodbye to Celtics fans via Twitter: "Will miss all my Boston fans I love y'all, u guys showed me love, E.House shoes tough to fill and y'all made it EZ for me to fill em."

"It's a difficult time when you see one of your good friends and teammates go," said Celtics captain Paul Pierce, after the morning shootaround. "You're happy when things don't happen and the deadline passes. That's probably the most difficult thing about this business, seeing a friend and teammate go, but you have to understand it's a business and things of that nature happen."

The Celtics are also sending rookies Luke Harangody and Semih Erden to Cleveland for a second round pick.

"I'm surprised, but it is what it is," Erden said. "I thank the Celtics fans. This is my first NBA trade, so I don't know."

Krstic is a perimeter center and had struggled with the Thunder this season. He averaged 7.6 points and 4.4 rebounds in 21 minutes.