The Saints defense had terrible breakdowns during four back-breaking big plays by the Rams (all in the first half). Meanwhile, the Saints didn't make any big plays of their own (zero turnovers for the third straight game and zero sacks).
Here are my observations after reviewing the tape:
Lowest point: The Saints' worst defensive play of the game came early -- when they gave up a 31-yard touchdown pass to tight end/fullback Cory Harkey. The whole play was a disaster. The Saints sent five or six defenders on a blitz (depending on whether outside linebacker Junior Galette was supposed to be rushing or covering Harkey -- which was hard to tell from the replay). Either way, Harkey was wide open after running into the flat from the fullback position, and quarterback Kellen Clemens threw the ball before any blitz pressure came anywhere close to him.
Then things got worse. Linebacker Curtis Lofton and safety Malcolm Jenkins appeared to have Harkey pinned against the sideline short of the first-down marker. But Lofton got taken out with a great block by tight end Jared Cook, and Jenkins missed the open-field tackle while diving low at Harkey's waist/hips. Saints cornerback Corey White was then waiting a few yards down the field -- but he looked unprepared to make the tackle (perhaps surprised by Jenkins' miss). White didn't have much momentum behind him as he tried to shove Harkey out of bounds, and Harkey barely moved as he kept running forward. Cornerback Keenan Lewis also tried to shove Harkey out of bounds at the end of the run, but Lewis was being blocked, so it was a lost cause.
Run over: Later in the first half, the Saints gave up runs of 29 yards and 40 yards to Rams rookie running back Zac Stacy (the first one setting up a touchdown and the second one scoring a touchdown). They were also huge miscues by the defense -- but at least both of those plays should be credited to some perfectly-executed blocks by the Rams, too.
Both plays came around the right end, and both were designed to have tight end Lance Kendricks take out Saints defensive end Akiem Hicks with a crack-back block at the line of scrimmage. Then Stacy followed pulling right tackle Joe Barksdale and pulling right guard Rodger Saffold around the corner.
On the 29-yard run, Barksdale, Saffold and tight end Jared Cook all combined to flatten safety Roman Harper to the ground at the line of scrimmage. Then Saffold and Cook also knocked linebacker David Hawthorne to the side. Stacy then hurdled some of the fallen bodies while breaking into the open field. And to make matters worse, Jenkins was flagged an extra 14 yards for a late shove out of bounds at the end of the run.
On the 40-yard run, the Saints were in a different defensive front, but the Rams' blocking scheme was just as effective. Kendricks took out Hicks, Saffold took out Lofton, and Barksdale ran into outside linebacker Parys Haralson. Haralson actually did a good job fighting off the block, but then he missed the tackle against Stacy as he came around the corner. Stacy then flashed some impressive speed while shooting past Jenkins, who took too shallow of an angle at him. After that, no one had a chance to catch Stacy.
No pressure: The Saints got virtually zero pressure on Clemens all day (though he only dropped back to pass a total of 20 times). And they were burned several times when they tried to blitz. Harkey's touchdown came against a blitz. So did Clemens' 4-yard touchdown pass to Kendricks later in the first quarter. And so did Clemens' 31-yard pass to receiver Chris Givens on a third-and-5 play that kept alive another TD drive in the second quarter.
Kendricks broke wide open on his touchdown pass. It appeared to be a mistake by linebacker Will Herring, who bumped Kendricks at the line of scrimmage, then let him go as if he expected help from behind. But it also could have been a missed assignment by Lewis or Jenkins, who both followed their men on slant routes to the middle of the field.
The 31-yard pass to Givens was clearly a mistake by White, who fell down while chasing Givens on a short route across the middle of the field. After White fell, Givens had clear sailing for another 20 yards. Then to make things a little worse, safety Isa Abdul-Quddus whiffed on an open-field tackle attempt toward the end of the play.
Caught off guard: The Saints' special teams were also caught off guard for a big play -- the Rams' surprise onside kick in the first quarter. The kick was about as perfectly-executed as you'll ever see. Kicker Greg Zuerlein's kick sailed high into the air, and receiver Stedman Bailey went up and caught it at its highest point as if it was a pass. Saints linebacker Ramon Humber was actually in position to take a swipe at the ball in the air, but he lost the jump ball to Bailey. Meanwhile, Hawthorne and running back Travaris Cadet weren't in position to make a play after initially running away from the play (though it's unclear if they could have made a difference regardless).
Crying foul: The Saints made things worse on themselves with three personal-foul penalties in the first quarter. I thought the first two were a little iffy (Lofton hitting Clemens a tad late when Clemens slid at the end of a read-option keeper, then Jenkins' late hit against Stacy). Although Jenkins clearly shoved Stacy after he was already out of bounds, Stacy also might have gotten away with a stiff-arm against Jenkins' face mask leading up to it.
The last penalty, however, was clearly a bad call. Jenkins was flagged for launching into receiver Austin Pettis, but Pettis got hit in the shoulders, and Jenkins made the hit with his hands and shoulder. There was no helmet contact from either player.
Something good: Aside from those game-breaking runs by Stacy, the Saints' run defense actually played well for much of the game. The Rams had a total of 13 runs and two short passes that gained 1 yard or less -- five that went for negative yards. Hawthorne was particular good, making several open-field tackles on run plays and short passes (finishing with 10 solo tackles). Lofton, Hicks and safety Kenny Vaccaro also had one or two nice solo run stuffs as well.