A week after facing Kansas City’s Alex Smith, an opponent they know like family, the 49ers will face a quarterback they wouldn’t recognize if he strolled in their locker room.

Ever heard of Austin Davis? No worries. You’re not alone if you’re clueless.

Here’s defensive coordinator Vic Fangio when asked if he knew anything about the Rams quarterback before last week: “I did not personally,” Fangio said.

Of course, Fangio and his players have since learned plenty about Davis, 25, in advance of their visit to St. Louis (1-3) on Monday night.

And here’s what they’ve discovered: The former undrafted free agent who entered the NFL in 2012 and was released twice last year looks like, well, a legitimate NFL quarterback.

Three starts into his career, Davis ranks seventh in the NFL in completion percentage (67.8), eighth in yards per attempt (7.9) and 10th in passer rating (96.8). For comparison, 49ers quarterback Colin Kaepernick ranks 16th in each of those categories. With Davis, the Rams are 1-2, but have averaged 26 points and 417.7 yards a game.

It’s safe to say non one saw this coming. Davis was released from the Dolphins’ practice squad 13 months ago and spent five weeks unemployed before he was signed by the Rams. He was a third-stringer in training camp this summer.

“He’s there for a reason,” said 49ers safety Eric Reid, “despite where he started the season.”

Davis became the next man up after injuries to Sam Bradford and backup Shaun Hill. Bradford sustained a season-ending ACL tear in the preseason and Hill was knocked out of the season opener with a calf injury.

Hill is now healthy, but Davis has claimed the starting spot by posting a 100.6 rating in his three starts. Last week, he threw for 252 yards and two touchdowns in the final two quarters and the Rams nearly erased a 27-point second-half deficit in a 34-28 loss to the Eagles. Jim Harbaugh said Davis’ performance had him unexpectedly riveted.

“It was one of the rare times where I’ve watched coaches’ tape and been excited about the game,” Harbaugh said. “… This was a quarterback that was making throws, an offense that was making plays. And then the last drive, they had a chance to win the game and I’m like, ‘Maybe they got the score wrong. Maybe the Rams really won this game.’”

Davis’ next start, however, will be his most daunting. Davis has started against the Buccaneers (30th in yards allowed per game), Cowboys (21st) and Eagles (28th). The 49ers rank second in total defense and opposing quarterbacks have a 75.7 passer rating against them, the third-lowest in the NFL.

“I think they’re probably the best all-around defense we’ve seen so far,” Davis said. “Huge, huge challenge for us and I think we’re ready for it.”

Davis’ early NFL career arc mirrors his nondescript beginnings at Southern Mississippi, where he was a walk-on who redshirted his first season. However, Davis became a four-year starter who broke Brett Favre’s records for career touchdowns, passing yards and completion percentage.

It’s premature to forecast similar success in the NFL, but Davis does possess the necessary size (6-foot-2, 221 pounds) and arm strength. On attempts 20-plus yards downfield, he’s completed 11 of 22 passes for 346 yards with two touchdowns and one interception. His 59.1 completion percentage on deep throws ranks third in the NFL, according to Pro Football Focus.

In addition to his physical tools, Fangio believes he has the proper makeup: “The game doesn’t seem too big for him,” he said, “for a guy that’s never played until three weeks ago.”

NFL history is dotted with flash-in-the-pan quarterbacks who fade back into obscurity once defenses figure them out. Just ask the 49ers. In 2010, Troy Smith came off the bench and posted a 116.6 rating to lead the 49ers, previously 1-6, to two straight wins. By 2011, though, Smith was playing in the now-defunct United Football League.

On the flip side, a former sixth-round pick came off the bench in 2001 and began putting together an improbable Hall-of-Fame career.

Don’t defenses eventually solve out-of-nowhere quarterbacks like Davis?

“Sometimes,” Fangio said. “They never caught up with Tom Brady.”