The Warriors knew of the surgery before Durant's announcement but allowed him to share the news himself, sources told The Athletic's Sam Amick.
The All-Star forward underwent an MRI in New York on Tuesday, which confirmed the injury, the team said in a release.
President of basketball operations Bob Myers said his club was cautious with Durant's rehab and consulted multiple experts before the forward was given the green light to suit up Monday. He also restated that Durant's initial injury was to his calf and not his Achilles.
Head coach Steve Kerr echoed Myers' comments during Wednesday's media availability.
"Kevin checked all the boxes and was cleared to play by everybody involved ... our feeling was that the worst thing that could happen would be a re-injury of the calf ... the Achilles came as a complete shock," Kerr told reporters, including The Athletic's Blake Murphy.
"If we knew this was in the realm of possibility, there's no way we would have allowed Kevin to come back."
Durant has a $31.5-million player option for 2019-20 season, which he could opt out of to enter unrestricted free agency this summer. Multiple teams - including the Los Angeles Lakers and New York Knicks - will reportedly continue their pursuit of Durant despite the injury.
The two-time Finals MVP came out firing in his first appearance since Game 5 of the Western Conference semifinals against the Houston Rockets. Durant had 11 points on 3-of-5 shooting - including a perfect 3-for-3 from distance - in 12 minutes of action.
He averaged 32.3 points per game on 51.4 percent shooting across 12 postseason appearances.
The series shifts back to Oracle Arena on Thursday for Game 6.