AT&T boss Randall Stephenson defended his decision to pick a controversial insider as his successor, days after billionaire Paul Singer’s activist hedge fund questioned the move.

Facing criticism over his decision this month to promote John Stankey, the divisive boss of AT&T’s entertainment unit WarnerMedia, to the role of operating chief, Stephenson said Tuesday the move is central to the company’s strategy.

“It was a very short list, and John Stankey quickly rose to the top,” the chief executive said at a media conference hosted by Goldman Sachs in New York.

As reported by The Post, Stephenson’s No. 2 has bulldozed the executive structure at WarnerMedia, formerly known as Time Warner, since AT&T acquired the company last year. Top execs like HBO CEO Richard Plepler and Turner president David Levy have left the division since Stankey took the helm.

But Stephenson said his wingman has been “breaking down silos” in the company as it gets ready to launch HBO Max, AT&T’s streaming service, which is meant to compete with Netflix and streaming services from Amazon, Disney, Apple and others.

HBO Max is in the process of beefing up its lineup and inked a multimillion dollar, five-year deal for the exclusive rights to stream “The Big Bang Theory.” Recently, the company shelled out $425 million for “Friends.”

Stephenson’s response came a week after Singer’s hedge fund Elliott Management Corp. revealed a $3.2 billion stake in the Dallas-based telecom firm and called for it to consider dumping several businesses it took on over the last five years, such as DirecTV.

The hedge fund pointed to the satellite TV provider, which has hemorrhaged subscribers, to the tune of nearly 1 million in the most recent second quarter. Elliott was spurred to action by Stankey’s elevation and potential succession to the role of CEO, according to reports.

Stephenson, who has been CEO for 12 years, didn’t address the succession plan but he did offer that Stankey has the media credentials and if he is able to turn HBO Max into a success, he is in good standing to take his job.

“Is he the heir apparent? The board hasn’t informed me I’m retiring yet,” Stephenson said. “If Stankey executes this plan, he’s well positioned.”

Inside the company, employees have told The Post that there’s confusion about the video strategy and that Stankey is not someone who takes constructive criticism — or any criticism.

Others say it’s too early to tell how HBO Max will fare against newcomers like Disney and Apple, both of which aggressively priced their services, at $6.99 and $4.99 a month, respectively.

AT&T plans to unveil a first look of HBO Max and reveal pricing details for the service on Oct. 29. HBO Max will launch early next year.