The appointment of Mr. Greenblatt allows Warner Media to speed up its plans for a streaming business, a process that was hampered under the older structure, Mr. Stankey said.

“We had to put a bit of a Band-Aid organization on top of things before we made this change,” he said. “Still, I think we’ve done a remarkable job with what we had. The new combined WarnerMedia will be much more successful filling in the gaps.“

AT&T was handcuffed until recently as it fought a lawsuit brought by the Trump administration’s Justice Department to block the merger. The company agreed to take a hands-off approach to its Turner division, which includes CNN and TNT, until the appeal was concluded. AT&T beat back that challenge last week, giving the company free rein.

Concerning the resignations of Mr. Plepler, who had been at HBO for 27 years, and Mr. Levy, who had spent 32 years at Turner Broadcasting, Mr. Stankey said, “I was disappointed but I don’t know that I was completely surprised.” He added, “When you have someone who had a tremendous amount of autonomy, they tend to covet that.”

Mr. Stankey plans to be a much more active steward of Warner Media than its executives had anticipated, three people familiar with the new AT&T said. He has started to consolidate advertising, distribution and production departments across Turner and HBO, as well as some of the back-end functions like human resources, accounting and technology. Those moves effectively took away large chunks of matters once overseen by Mr. Plepler and Mr. Levy, the people said.

In Mr. Stankey’s view, Warner Media needed to reorganize the TV networks in a way “where they could cooperate more together,” he said, which meant that some top executives would have reduced roles. He said his discussions with Mr. Plepler and Mr. Levy, which took place separately, never got far enough to explore what roles they each might accept.

Going up against Netflix and Disney will be tough. Mr. Stankey acknowledged that a household is likely to pay for just four or five of these services, and that AT&T will have to prove its worth to consumers.