Investors have been concerned that increased competition from streaming services could cut significantly into the cable industries profits. But sports have been the exception, as most live sporting events were broadcast primarily on cable networks.
Now, that assurance in viewing has weakened. Sports viewing habits have shifted as most fans say they would subscribe to a streaming or over-the-top sports channel. And more digital streaming services provide live coverage online.
"I don't think that sports are immune from cord cutting," Costolo told CNBC's "Squawk Alley." "And, in fact, I think you saw Bob Iger in the last Disney earnings announcement make comments about some of the over the top offerings like Sony View now licensing their content."
NBC this month made 4,500 hours of Olympics coverage available on its website and sports app. Many of the events require a cable or satellite TV subscription, but fans can sign up for access via Sony's PlayStation Vue service.
Earlier this month, ESPN's owner Walt Disney's said it would pay it would pay $1 billion for a 33 percent stake in BAMTech, the video streaming unit of Major League Baseball. BAMTech and ESPN will work together to create a standalone streaming service.
Costolo said that media leaders acknowledge the shift and are already working to deal with it.
"So, I think you'll see continued fragmentation in the space," he said.
— Disclosure: CNBC parent NBCUniversal owns NBC Sports and NBC Olympics.