While Falcons coach Arthur Smith didn't want to speculate on the exact nature of McCarron's injury until after the veteran has an MRI, he did not sound encouraged by the possible prognosis.
"I really feel for AJ McCarron," Smith said. "Any time something like that happens and it's non-contact, it's tough when you know what these guys put into it. I feel for AJ. We'll obviously get the MRI to confirm but he's heartbroken.
"He had bounced back and he was out there fighting, fighting for his team and trying to get the extra yards."
The injury came with 10 minutes left in the first half. McCarron handed the ball off to running back Qadree Ollison and as he came out of his typical movement following the handoff he grabbed his right knee.
McCarron tried to walk toward the sideline but couldn't initially get there. Falcons medical staff came to examine him and he walked off to the sideline. Minutes later, McCarron walked slowly to the Atlanta locker room. He completed 3-of-6 passes for 43 yards and had one run for seven yards against the Dolphins. The run came a play before he hurt his knee.
Smith said he wasn't sure whether that play had anything to do with McCarron's injury. McCarron was replaced by undrafted rookie Feleipe Franks, who he had been battling with for the No. 2 position throughout training camp. Franks, who was initially expected to play the second half, entered the game with 10 minutes left in the first half instead.
Franks completed 4-of-9 passes for 46 yards, no touchdowns and one interception while also rushing three times for 32 yards. He was sacked four times.
"The one thing that Feleipe has shown is that he can extend plays," Smith said. "I thought he threw the ball a little bit better tonight at times. Still got to push him where he's had a lot of growth and will continue to grow."
Smith said in order to develop quarterbacks, you have to play them and that was the philosophy with Franks over the first two preseason games. Smith said unfortunately the situation Franks faced Saturday -- being forced to play earlier than expected due to injury -- is what his role would be as a backup quarterback sometimes.
McCarron's injury leaves Atlanta with more questions behind starter Matt Ryan, who has not played this preseason. The Falcons could search the free-agent market, where former first-round picks Blake Bortles and Robert Griffin III are among the top available options.
"You're always going to look to improve the team," Smith said. "So we have to assess what the injury is. Until we get an MRI, you've seen things happen before and it's quick. But any time, we're always looking to improve the roster.
"If its long term, sure, we'll continue to look."
The Falcons signed McCarron to a one-year deal for more than $1.21 million on April 30 -- hours prior to the second day of the NFL draft -- to be No. 2 on the depth chart. A fifth-round pick out of Alabama in 2014 by Cincinnati, McCarron has played in 17 games, starting four and completing 62.6 percent of his passes for 1,173 yards, six touchdowns and three interceptions.