This is my Saturday column.

For a while, Eric Mangini was a joke.

People called him “The Mangenius.” Said things like, “Oh look, the Mangenius is a nerd who can’t relate to his players. Ha ha ha,” or “Oh look, the Mangenius is a fraud who can’t coach without Bill Belichick. Ha ha ha,” or “Oh look, the Mangenius just got fired again. Ha ha ha.”

All of these comments were ironic, probably even sarcastic because Mangini isn’t supposed to be a genius. He’s a loser. Ha ha ha!

The joke ended. I’ve seen this guy this guy coach, and I admire him. You should admire him, too.

Look what he did Sunday. The 49ers’ defensive coordinator held the fifth-ranked Atlanta Falcons offense to 16 points, even though he didn’t have his top three cornerbacks — Tramaine Brock, Kenneth Acker and Keith Reaser. They were all hurt.

Mangini had to play fourth-stringer Dontae Johnson, and two corners the Niners called up from the practice squad just a few days before the game — Marcus Cromartie and Chris Davis. They shut down quarterback Matt Ryan, and the defense won the game for San Francisco.

Could former 49ers defensive coordinator Vic Fangio have prepared those three backups as well in such a short amount of time? I don’t think so, and I consider Fangio the best defensive coordinator in the NFL.

Fangio specializes in coaching linebackers — he was a linebackers coach in the NFL for nine seasons. Mangini specializes in coaching defensive backs. He was a defensive backs coach with the New England Patriots for five seasons before he became their defensive coordinator for one season, and then became the head coach of the New York Jets, followed by the Cleveland Browns.

Mangini and defensive backs coach Tim Lewis have done a masterful job with the 49ers secondary this season. Most of the defensive backs are young players learning a brand-new system, and yet every single one — from corners to safeties to starters to backups to practice squad guys — has played well.

For the first time in a long time, the strength of the 49ers’ defense is the secondary, not Jim Tomsula’s front seven.

Is Mangini perfect? No, of course not. But, he has imperfect tools. He has no Justin Smith, no Ray McDonald, no Chris Culliver, no Perrish Cox, no Chris Borland, no Aldon Smith, no Patrick Willis. Could Fangio have done any better without this group of players? Could Fangio have held the Falcons to 16 points like Mangini did? Could Fangio have held the Packers to 17 points like Mangini did?

Probably not. Mangini deserves recognition.

This guy had to eat crow for years. Had to leave the league after the Browns fired him as head coach. Had to work for ESPN as a television analyst for two years. Had to work as a “senior offensive consultant” with the Niners for one year, whatever that means. And then, he got promoted to tight ends coach last season, not much of a promotion.

He worked back up from the bottom and kept his mouth shut. None of this eating crow destroyed Mangini. Now, when he speaks at press conferences he seems confident. He smiles. And he isn’t apologetic or embarrassed about his past. He faces it head-on because he has a head coach’s demeanor. Call it grace under fire.

No one else on the 49ers has a head coach’s demeanor. Certainly not head coach Jim Tomsula. He has a plumber’s demeanor. He has been miscast as a head coach. Until this season, his vision was limited to the line of scrimmage. Mangini sees the whole field.

Mangini has to work for a dysfunctional head coach, a dysfunctional coaching staff, a dysfunctional team and a dysfunctional organization. And yet, he is excelling. And not only that — he’s making the case he’s a good NFL defensive coordinator, and maybe someone who could be a head coach again in the near future.

Maybe he could be the 49ers’ head coach as soon as next season. At least he would have to be in the conversation if the Niners were to fire Tomsula. As opposed to offensive coordinator Geep Chryst, who’s in no conversation. Mangini forced himself into the conversation by the quality of his work.

It’s time to start talking seriously about Eric Mangini.

Grant Cohn writes sports columns and the “Inside the 49ers” blog for The Press Democrat’s website. You can reach him at grantcohn@gmail.com.