An MRI taken Sunday confirmed the hyperextension and also revealed a bone bruise, a source told ESPN's Dan Graziano. Martin is expected to be ready in time for the start of the regular season, the source told Graziano.
Martin suffered the injury in the second quarter, when Bengals cornerback Dre Kirkpatrick went low and saw his helmet hit Martin in the knee. Martin got up slowly and limped to the sideline. He did not return to the game.
In the fourth quarter, he was standing on the sideline.
"I'm not thinking that Zack is problematic here," Cowboys owner and general manager Jerry Jones said after the game.
Coach Jason Garrett said the team's medical staff was "somewhat optimistic based on the initial observation."
During the game, there was initial concern. Linebacker Sean Lee said the sideline was "devastated" when Martin went out of the game. As he was in the medical tent getting evaluated, Pro Bowl left tackle Tyron Smith and center Travis Frederick, as well as right tackle La'el Collins and running back Ezekiel Elliott, stood nearby waiting for Martin.
"You're definitely worried when you see him go down like that, but we also know how tough he is, and he's a guy that's going to push and play through things," said Frederick, who did not play because of stingers that kept him out of the final few practices in Oxnard, California. "So coming off, he got his evaluation, and then when he came out of the tent, he kind of gave us a little thumbs-up, and that made us feel a lot better. A big wave of relief."
Martin, who signed a six-year extension this offseason that included $40 million in guaranteed money, has not missed a game in his first four seasons. He has been named to the Pro Bowl each season and twice has been named a first-team All Pro.
Kadeem Edwards replaced Martin on Saturday. Frederick is not playing because of a series of stingers that led him to get a second opinion from specialist Dr. Robert Watkins during the week.
The Cowboys also lost starting safety Xavier Woods with a hamstring injury. He got hurt while nearly intercepting an Andy Dalton pass in the second quarter. The Cowboys are thin at the safety spot, and their other starter, Jeff Heath, is working through a low ankle sprain.
The Cowboys do not practice again until Monday and play Aug. 26 against the Arizona Cardinals.