Analysis | How can the Chiefs stop the 49ers? The Ravens provided a blueprint.

“There were some tells we picked up on with [tight end George] Kittle and [running back Christian] McCaffrey in pass protection, and figuring out what they were going to be doing and where they were going,” said one person involved in Baltimore’s preparation for that game, who spoke on the condition of anonymity because he is not permitted to divulge the specifics of game plans. “I’m not going to tell you what they were, because we might end up playing them again, but I can guarantee you Spags” — as people around the NFL refer to Chiefs defensive coordinator Steve Spagnuolo — “knows exactly what I’m talking about.

“He’s spent a lot of time studying that game film,” this person said, “and our defenses are very, very similar in a lot of ways. He’s talking to certain people, and [Chiefs defensive line coach] Joe Cullen has some strong relationships here, and he’s talking to everybody, too. They are going to copy some of the things we did to them. You can count on that.”

Lest you forgot, Purdy went from MVP favorite to relative afterthought in the betting markets after that 33-19 defeat to Baltimore on Christmas night. [A garbage time touchdown with Sam Darnold at quarterback made the final score closer than it might have been.] Purdy had not thrown more than two interceptions in a game all season before that meeting — and his previous two-interception games were both without future Hall of Fame left tackle Trent Williams in the lineup. He had just three picks in six home games before the Ravens arrived, but Baltimore confounded him, beat him up and picked him off four times, including on three of the 49ers’ first four possessions. He entered the game leading the NFL in passer rating, but he finished a foul 18 for 32 for 255 yards and a season-low 42.6 rating before leaving with a stinger. The 49ers allowed a season-high 25 pressures in that game — seven more than in any other game — and their quarterbacks were hit a season-high nine times, according to website TruMedia.

“That film will be invaluable to them,” said someone else involved in Baltimore’s preparations for that showdown, speaking under similar restrictions. “They have a lot of the same kind of pieces we do, except for [Ravens hybrid safety] Kyle Hamilton. Nobody else has that. But Spags has the personnel to hold up in man coverage on third down, he has corners who can get physical at the line of scrimmage and he’ll have a plan for Purdy.

“Purdy is so smart and he processes information so quickly, you really have to be willing to change up your looks quickly. Whether you are trying to cloud one side of the field, you need to change up that look, because he processes that first read so quickly. So you need to create some hesitation with him. A lot of [Shanahan’s] route concepts go inside-out, and there’s some things you can do to try to take away some of those quick throws.”

One of the ways the Ravens did that was by nearly doubling their use of cover-six from their season average. The coverage is a hybrid of cover-two and cover-four, flooding the intermediate passing areas with four defenders, with three deeper defenders behind them. It’s easy to toggle between concepts and deceive the opposing offense, and the Ravens ran cover-six 28 percent of the time in Week 16, according to TruMedia. (They only used it more often only once — the following week against Miami’s offense, which is derivative of Shanahan’s). The average NFL team employed cover-six just nine percent of the time this season (including playoffs). Baltimore’s use of cover-four (quarters coverage) also ticked up against Purdy.

Purdy, who finished with an NFL-best 113.0 passer rating in the regular season while averaging 9.6 yards per attempt, faced less cover-six than any other zone look in the regular season, and he struggled the most against it. In his past four games, including the loss to Baltimore, Purdy has averaged just 6.4 yards per attempt vs. cover-six, with a rating of 41.4. And in that same span, against cover-four, Purdy has a rating of 56.3, completing just 15 of 27 passes. In that span, Purdy has a rating of 127.9 vs. man coverage and 71.6 vs zone, when he’s had just two touchdowns, five picks and an average of 7.2 yards per attempt.

Including the playoffs, Spagnuolo has played the sixth-highest rate of man coverage in the NFL, relying heavily on two shutdown corners. Expect that to change Sunday. “Spags is going to be Spags, but it has to be within reason,” said the second person who helped Baltimore prepare. “There are some protection issues you can create, but you don’t have to bring six or seven guys to get there.”

“Spags loves to blitz, we all know that,” said one longtime NFL scout who has done advance work on the Chiefs, and spoke under the same restrictions. “But Baltimore’s simulated pressures are what had that kid’s head spinning. [Spagnuolo] had two weeks to get ready for this game and I think he’ll break with some tendencies here.” Mike Macdonald — the former Ravens defensive coordinator — “left him a blueprint.”

Whatever the Ravens gleaned from Shanahan’s usage of his running back and primary tight end in pass protection, the results were significant. The Ravens, who led the NFL in sacks, had their second-highest pressure rate of the season at San Francisco (49 percent), despite no increase in their blitz rate (21.6 percent, close to their season average). They shredded the right side of the 49ers’ offensive line in particular.

For all of the 49ers’ passing acumen, everyone I spoke to about this game believes it will ultimately be decided at the line of scrimmage, in the run game. The Chiefs have major issues stopping the run when a fullback is involved, and San Francisco has the most dynamic fullback in football, Kyle Juszczyk, who will rarely leave the field Sunday. Shanahan has lost many a big game by getting pass-happy, and it would be shocking if he, too, didn’t break tendency here.

“Kyle is going to run the ball and keep running the ball,” the second staffer told me. “That’s a two-back offense and he’s going to make Spags prove he can stop it and keep proving he can stop it. They love those alley runs and they’ll test the edges with heavy personnel and see how Spags reacts.

“[Juszczyk] is a huge part of everything they do offensively, and they can displace him and match him up from different positions and create whatever they want … with him on the field. It’s a problem and it asks a lot of your linebackers. Juice will take you to the ball, but even when you know what’s coming in that run game, it can be tough to slow down. There’s no doubt in my mind that Kyle will start this game committed to running the ball. Can you make it the kind of game where he has to deviate from that?”

Confounding and intercepting Purdy early, as the Ravens did, would certainly force the issue.