CHARLOTTE, N.C. -- The Carolina Panthers expect to be without backup quarterback Sam Darnold for an extended period of time with what coach Matt Rhule described as a "significant" high ankle sprain suffered in Friday night's preseason finale against the Buffalo Bills.

The typical recovery time is four to six weeks.

"It is a significant injury that could take some time," Rhule said Saturday. "If it's at least a four-game injury, we can put him on the short-term injured reserve."

Carolina might also have lost kicker Zane Gonzalez for the season with what Rhule described as a "significant, long-term" groin injury that he suffered while warming up on the sideline. General manager Scott Fitterer and special teams coordinator Chris Tabor already are looking for a replacement.

"We have to attack that," Rhule said. "That's a major thing for us."

Darnold underwent an MRI on Saturday and the team is seeking further opinions to determine the exact severity of the injury as it decides whether to place him on injured reserve. Darnold has to be on the 53-man roster that will be set Tuesday to be eligible for return from injured reserve.

The injury leaves the Panthers with two quarterbacks, starter Baker Mayfield and former XFL starter P.J. Walker, on the roster heading into the Sept. 11 opener against the Cleveland Browns.

Rhule said he is confident that Walker, who is 2-0 as a starter the past two seasons, can back up Mayfield while Darnold recovers. Rhule said offensive coordinator Ben McAdoo and quarterbacks coach Sean Ryan were "really pleased" with the way Walker played Friday after a rough outing the week before at New England.

Walker completed 8 of 10 passes for 50 yards and no turnovers after having two interceptions and a fumble recovered for a touchdown against the Patriots.

"When he's out there, guys play," Rhule said.

Darnold suffered his injury in the third quarter when he was hit from behind by Bills defensive tackle C.J. Brewer while scrambling to his right to throw the ball away. Replay showed that Brewer rolled over Darnold's left ankle.

It was the second time in a week that the Panthers lost a quarterback in a preseason game. Third-round pick Matt Corral suffered a season-ending Lisfranc tear against New England.

The Panthers had planned to keep three quarterbacks, with Corral expected to be the third behind Mayfield and Darnold. Corral's injury changed the mindset to keeping two, but Darnold's injury forces another adjustment.

Meanwhile, Mayfield looked solid in his final tuneup before facing his former team in the opener. He completed 9 of 15 passes for 89 yards and two touchdowns, all against Buffalo's second-string defense.

Mayfield finished the preseason 13-for-22 for 134 yards and two touchdowns.

"He protected the ball, kept the ball out of harm's way," Rhule said. "At the same time, he was not afraid to make aggressive throws."

Mayfield said he feels "great about this team" heading into the opener.

"I feel great about the guys up front and the weapons we have at the skill positions, and we haven't been out there as a complete unit yet," he said.

Running back Christian McCaffrey didn't play in any of the three preseason games to ensure a player who has missed 23 of his past 33 regular-season games entered the season healthy.

"I'm 100 percent confident in this group that we have going forward," Mayfield said.