The 31-year-old was making the fourth defence of his title but the home crowd at London’s Copper Box were left disappointed following a bruising majority decision defeat for the former Olympic champion.
One judge scored the fight as a draw at 114-114, but further scores of 115-112 and 116-112 in favour of the American challenger condemned DeGale to only the second defeat of his career.
DeGale told BT Sport 1: “Crazy, I can’t believe that. I’ve boxed at a high level for two-and-a-half years and I’ve got to go back to the drawing board, I’ve got to sort certain things out and change things.
“I thought I’d won it but I’ll have to look back. I’m going to have to go back now and sort out certain things.”
DeGale underwent surgery on his right shoulder following his controversial draw with Badou Jack in January but after a steady start he was rattled in the fifth round when Truax unleashed a number of fierce shots which left blood pouring from his nose.
Asked whether his shoulder problems had contributed to the outcome, DeGale added: “There’s no pain there, I’ve probably rushed back to be honest… maybe I should have waited till next year.
“I’m really gutted, I’m so upset. I want the rematch, that’s what I want.”
One of DeGale’s British super-middleweight rivals, Chris Eubank Jr, was quick to have his say on Twitter, saying the 31-year-old had “let down British boxing”.
The 34-year-old Truax was thrilled to have pulled off his gameplan.
“I want to thank everyone for the opportunity,” he said. “We noticed that he didn’t pressure that much, he’s a helluva boxer, a classy boxer, I couldn’t box because he’s a better boxer than me but I had to make it into a dogfight and that’s what I did.”
Earlier Lee Selby called for a showdown with fellow Briton Josh Warrington after successfully defending his IBF world featherweight title against Eduardo Ramirez.
The Welshman secured a unanimous points victory over Ramirez, although his belt was not on the line after the Mexican failed to make the weight.
England’s Warrington is Selby’s mandatory challenger after beating Dennis Ceylan in October, and Selby believes the fight would be good for British boxing.
“First of all it’s a massive fight,” Selby said. “He has earned the mandatory position so to get there he has to be a good fighter.
“We’ve sort of exchanged words in the past but a fight between Josh and I would be massive and fantastic for British boxing.”
George Groves, who inflicted the other defeat of DeGale’s career, followed Eubank Jr in criticising his rival.