Aaron Glenn spent his first year as an assistant coach working down the hall from the one of the best offensive minds in the NFL.

Glenn, the Detroit Lions’ third-year defensive coordinator, was an assistant secondary coach with the Cleveland Browns in 2014 when Kyle Shanahan was the team’s offensive coordinator.

As head coach of the San Francisco 49ers, Shanahan’s teams have finished sixth or better in scoring in the NFL three of the past four seasons. He coached Matt Ryan to an MVP season and helped the Atlanta Falcons reach the Super Bowl in 2016. And with the Browns, Shanahan made Johnny Manziel look like a serviceable quarterback.

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Glenn, who’ll match wits with Shanahan when the Lions play the 49ers in Sunday’s NFC championship game at Levi’s Stadium in Santa Clara, California, said he learned one very important lesson about Shanahan’s coaching during their season together in Cleveland.

“He spends a lot of time of understanding what fronts you’re in and trying to attack the fronts you’re in,” Glenn said. “He’s one of the best in the league at doing that, too, now. I’ve seen that personally on how much time he spends with that. And then I’ve seen him grow as far as his pass concepts, especially in how today’s game works right now. So, it’s going to be a task, especially with the players they do have.”

Thos players comprise one of the most star-studded offensive rosters in the NFL, with first-team All-Pros at left tackle (Trent Williams), tight end (George Kittle) and fullback (Kyle Juszczyk), presumptive Offensive Player of the Year Christian McCaffrey at running back, and two dangerous receivers in Deebo Samuel and Brandon Aiyuk.

That group helped turn quarterback Brock Purdy — "Mr. Irrelevant" as the final pick in the 2022 NFL draft — into an MVP candidate this season. And while the 49ers led the league in yards per pass (8.9) and pass plays over 20 and 40 yards (75 and 15, respectively), Lions coach Dan Campbell said the key to stopping their offense starts with suffocating the run.

“You’ve got to stop the run because if you don’t, they’ll rush for 250 on you and then they won’t even worry about passing,” Campbell said. “Everything has to start there.”

Shanahan’s offenses have long featured some of the best rushing attacks in the NFL, and the run game was supercharged last year with the addition of McCaffrey via a trade with the Carolina Panthers.

The best dual-threat back in the NFL, McCaffrey led the league in rushing yards (1,459) and total touchdowns (21) this season and has ben a DNA match for Shanahan’s wide zone scheme.

“He’s got good vision and he’s patient,” Lions defensive tackle Alim McNeill said. “He’s a veteran running back that’s been doing it for a while, but once he sees something he’ll put his foot in the ground, he’s north and south straight to the end zone. He doesn’t do too much dancing and stuff, so he likes to get downhill and get to it.”

Campbell said Shanahan’s offense has evolved in recent years to what he called a “fast-flock motion.”

The 49ers use multiple personnel groups and pre-snap motions to create mismatches, and Campbell said once defenses adjust to the confusion they create, they throw more wrenches in the mix with misdirection plays and run-action passes.

“Any front you get into, they adjust on the fly because you do it, you just rep it, rep it and they’re really good at it,” Campbell said. “So you get enough of that, then you get the counter off of it. You get that and then you’ll get gap scheme back this way. You’ll get boot back this way, then you get play-pass this way. You pull the guard and it’s hard action and here comes the dagger, here goes the post over the top. He just does an unbelievable job of setting you up. He’s going to make you stop it, and if you don’t stop it, you keep getting it. And then once you feel like you’re about to stop it, he counters. And he’s just always been great about that, the way he sets it up.”

The Lions finished second in the NFL in rush defense at 88.8 yards per game in the regular season and have been even stingier in the playoffs (78.5 yards a game), though they allowed 5.9 yards per carry in their divisional-round win over the Tampa Bay Buccaneers last week.

McNeill has played well in the middle of the defensive line after missing a month with a knee injury, and the Lions have gotten solid run defense on the edges from Brian Branch and their other defensive backs most of the season.

“I feel like we’ve been detailed with it,” McNeill said. “We’ve been using our hands, we’ve been getting off blocks, we’ve been making plays in the backfield so I feel we’ve been aggressive, really aggressive.”

To stop a 49ers team that finished third in the league this season at 28.9 points per game, the Lions will need that detailed, aggressive approach and more Sunday.

Campbell said Shanahan has no match as the best offensive coordinator in the NFL — “He’s tops,” Campbell said — while Glenn said his defense has “our hands full” playing the most balanced offense in the league.

“You’ve got your hands full in both regards,” Campbell said. “And Purdy does a hell of a job — they throw a lot of daggers middle of the field and he does a hell of a job with touch, timing, rhythm. But we have to stop this run game. It just has to start there. As much as you can, you have to try to make this team one-dimensional and that’s not easy to do.”

Contact Dave Birkett at dbirkett@freepress.com. Follow him on Twitter @davebirkett.

This article originally appeared on Detroit Free Press: Detroit Lions plan to limit 49ers attack: 'You've got to stop the run'