Thearon W. Henderson / Stringer

That’s the worst loss the 49ers have suffered in Kyle Shanahan’s tenure. A 43-17 loss to the Miami Dolphins at home. It was supposedly a scheduled win, and realistically, it’s the worst loss this team has suffered since the end of the Jim Harbaugh era. The defending NFC champions are now 2-3 after the easiest part of their schedule.

To be sure, there have been more lopsided losses than the one doled out by the Miami Dolphins on Sunday.

Against the Seattle Seahawks in 2018: a 43-16 battering.

Against the Dallas Cowboys in 2017: a 40-10 smackdown.

But neither of those losses, nor the 25 often horrendous ones endured in the Jim Tomsula and Chip Kelly eras really compare, because the expectations were not the same. Jimmy Garoppolo was either injured, not yet on the team, or the coaches were, well, inept.

The last time the 49ers really had this feeling? 2014.

The end of the Jim Harbaugh era.

This is not a suggestion in the slightest that this is where the 49ers are heading. Harbaugh’s grating approach was not endearing, nor sustainable in the long term, and Kyle Shanahan is discernibly well-loved and respected by his team.

But Sunday marked the first time since October 19, 2014, when the 49ers, coming off a Super Bowl loss, had their asses handed to them by the Denver Broncos in a 42-17 loss.

Those Broncos went to the Super Bowl, though.

This Miami Dolphins team, at 2-3, with Ryan Fitzpatrick at quarterback, does not seem likely to be destined for the same trajectory. Sunday’s game was one in which the 49ers were supposed to win, and handily, judging by the -9.5-point spread that they were given.

So what happened?

Most of the warning signs were there, but two factors came as a surprise. The obvious factors were explained in-depth earlier this week in KNBR’s quarter-season review, and they showed up again on Sunday.

• Offensive line play: This offensive line has been inept this season. Their run blocking is sub par, with pull and second-level blocks not being executed with efficiency, and ground being given up consistently. But the pass blocking has been the real shocker, with the 49ers allowing a league-leading 50 pressures and 28 quarterback hits entering the game. It wasn’t as much the one-on-one bullying that happened so frequently against the Eagles as it was the miscommunications and lack of communication. On the first sack of the game, Trent Williams thought Laken Tomlinson was there to take his man. Tomlinson was not. This was and is a common thread that was part of the reason Jimmy Garoppolo was pulled; in order to “protect” him, as head coach Kyle Shanahan said at halftime. The 49ers allowed five sacks and eight quarterback hits on Sunday, right in line with their season average. Below is what those numbers look like compared to last year.

• Pressures, not sacks, and bailouts: Without Nick Bosa, Dee Ford and Solomon Thomas, there’s been a serious lack of execution on the defensive side of the ball. Entering Sunday’s game, the 49ers pressured quarterbacks 30.3 percent of the time, good for 3rd in the NFL. But they had just eight sacks and a 5.7 percent sack percentage, which is 16th-best in the league. There were just two sacks accrued on Sunday, and five quarterback hits.

The running game was somewhat functional with Raheem Mostert back, as you’d expect. He had 11 attempts for 90 yards, but was limited by everything else collapsing around him.

Where the 49ers really went wrong was at quarterback and corner. Jimmy Garoppolo looked like Nick Mullens did last week, if not worse. He overthrew his receivers in egregious fashion and lofted a pair of pop-ups to be easily intercepted, finishing 7-of-17 with 77 yards and two interceptions. Again, embarrassing interceptions:

Jimmy Garoppolo keeps throwing INTs. 15.7 QB Rating.

Ryan Fitzpatrick is outplaying him badly, 150 QB Rating at halftime.pic.twitter.com/4Cf4RcBKE1

— Dov Kleiman (@NFL_DovKleiman) October 11, 2020

It seemed like Garoppolo’s return could get this team back in gear. He’s efficient at handoffs, and in theory, could have re-instated the play-action passing success the 49ers had attacked teams with all last year. Instead, he was inept. If this team has any hope of success, that cannot happen again. His performance was shocking.

Also shocking was the 49ers’ decision at corner. Ahkello Witherspoon, who was questionable with a hamstring injury, was not seen running full speed before the game, but he was made active. With Richard Sherman, Emmanuel Moseley and Dontae Johnson out, the 49ers opted to go with practice squad activation Brian Allen, rather than Witherspoon or Ken Webster, the team’s other option at corner.

The Dolphins targeted him early and often, and by the time he was pulled for Witherspoon, the Dolphins had a 21-7 lead, and Allen had allowed the following:

139 receiving yards

31 penalty yards on two penalties (horse collar and defensive pass interference)

2 receiving touchdowns

It begs the question why he was not pulled sooner and why Witherspoon did not start. It is among the many questions the 49ers will have to ask themselves after what is by far the worst performance in recent memory.