McCarron suffered a hairline fracture of his right collarbone in Friday's 19-17 win over the Cleveland Browns and it has yet to be determined how long he will be sidelined.
Buffalo entered training camp with McCarron competing for the starting job with Nathan Peterman and rookie Josh Allen, the team's first-round pick in the NFL Draft.
"All I really know right now is that he hurt his right shoulder," Bills coach Sean McDermott told reporters of McCarron's injury following Thursday's game. "We'll get some more tests done when we get back to Buffalo."
Allen, who was taken with the No. 7 overall pick by Buffalo, and Peterman did nothing to diminish their chances against the Browns.
Allen completed 9 of 13 passes for 60 yards and a 2-yard touchdown pass to wide receiver Rod Streater while guiding the Bills' offense on three possessions.
It was an upgrade from Allen's preseason debut, when he finished 9 of 19 for 116 yards and a touchdown against the Carolina Panthers.
"I thought Josh came in and moved the ball. He was productive," said McDermott of Allen's play. "Three scoring drives I believe and then Nate did some good things as well."
Peterman, who started the preseason opener, also was efficient by completing 8 of 10 passes for 113 yards, including a go-ahead 35-yard scoring strike to Nick O'Leary in the fourth quarter.
"In particular down the stretch, I thought he made the guys around him better, which was good to see," McDermott said of Peterman.
The 6-foot-5, 237-pound Allen threw for 1,966 yards with 19 touchdowns last season at Wyoming. Peterman appeared in four games with Buffalo in 2017, finishing 24 of 49 for 252 yards with two touchdowns versus five interceptions.
Allen acknowledged there is plenty of room for growth in his game but was more concerned with McCarron's status after the game.
"Whatever happened tonight with AJ it's some tough circumstances because anybody that knows AJ knows he's a great dude," said Allen. "I'm hurting for him. I sent him a text. You hate to see it happen, but I know there are a lot of things to improve on in my game and I'm going to continue to do that."
McCarron, 27, signed a two-year, $10 million contract with Buffalo in March after four seasons with the Cincinnati Bengals.
He has played in 11 regular-season games, including three starts. All three of those starts came in 2015, and the Bengals had a 2-1 record in those games.
McCarron played in three games last season and completed 7 of 14 passes with no touchdowns and no interceptions.