Despite Baker Mayfield's struggles with the NFL's lowest-ranked offense, the Carolina Panthers will not be able to turn to Sam Darnold any time soon.

Darnold is eligible to be activated off injured reserve Monday but is not close to being fully recovered from the high ankle sprain he suffered in late August, a source told ESPN's Adam Schefter.

Mayfield has the NFL's lowest QBR through four weeks at 15.4 and his 55.4 completion percentage is third-worst in the league, only better than two 2021 first-round draft picks -- the Chicago Bears' Justin Fields (50.7) and the San Francisco 49ers' Trey Lance (48.4), who is out for the season with an ankle injury.

Sunday's 26-16 loss to the Arizona Cardinals dropped the Panthers to 1-3. Mayfield committed three turnovers, throwing two interceptions and losing a fumble.

Mayfield, the top overall pick in the 2018 draft, has thrown for 747 yards with four touchdowns and three interceptions this season after winning a training camp competition with Darnold, the No. 3 overall pick in 2018, for the starting quarterback job.

Darnold suffered his injury in the Panthers' preseason finale against the Buffalo Bills. Darnold had his own struggles as the Panthers' starting quarterback last season, when his total QBR of 33.2, ranked 29th among qualified quarterbacks.

With Darnold and 2022 draft pick Matt Corral, who is out for the season with a Lisfranc injury, on IR, PJ Walker is the only other active quarterback on the Panthers' roster.