Juan Castillo sat in his Halas Hall office Monday afternoon attempting to offer a witness’s explanation for what went wrong.

The Chicago Bears offensive line coach knew his troops were responsible for the fourth-and-1 malfunction that abruptly ended the offense’s opening drive Sunday at Nissan Stadium in Nashville, Tenn. It wasn’t all that complicated either.

Needing to gain a foot and a half to keep an encouraging possession alive, the Bears called a dive play to running back David Montgomery. Right guard Germain Ifedi and right tackle Rashaad Coward were tasked with double-teaming Tennessee Titans defensive lineman Teair Tart.

Yet what the Bears drew up and anticipated isn’t what unfolded. At the snap, Tart lunged to his left. Ifedi whiffed badly with his block. Coward struggled in his attempt to help.

Montgomery was hit within 0.7 seconds of touching the ball and barely got back to the line of scrimmage.

“Germain was trying to come off and blow the guy up in that gap,” Castillo said. “And when he stepped, he dropped his head and crossed over with the second step. When (Tart) slanted toward Rashaad, (Germain) really has to be able to help with his back shoulder to give Rashaad some body presence so they can create movement. But essentially what he was trying to do was to come off and blow the guy up, thinking he was going to stay in his gap.”

Whatever the explanation, all of it went haywire. Turnover on downs. Scoring threat squelched.

It was a tone-setting breakdown in a dispiriting 24-17 road loss.

After the game, Bears coach Matt Nagy expressed his agitation with not being able to execute in that situation. Fourth-and-1. With a basic play.

“It’s a dive, you know?” Nagy said. “It’s what we do in youth football.”

It was another example of how a bottom-tier offense seems to struggle with the basics, too often making the task of gaining 1 measly yard look like a grueling attempt to complete an American Ninja Warrior course.

For good measure, the Bears wasted another fourth-and-1 opportunity on their first possession of the second half, this time when rookie left guard Arlington Hambright committed a false-start penalty at the Titans 31. Tight end Jimmy Graham false started immediately after, and the self-inflicted 10-yard backpedal forced the Bears to punt.

Oh, brother.

Statistically speaking, the Bears’ short-yardage breakdowns might not be as extreme as they seem. The offense is 7-for-10 on third-and-1 this season and 2-for-4 on fourth-and-1, ranking in the middle of the league in conversion percentage in both situations.

The Bears have scored two touchdowns from the opponent’s 1-yard line: Javon Wims’ catch in the fourth-quarter comeback against the Detroit Lions in Week 1 and quarterback Nick Foles’ third-down sneak against the Carolina Panthers in Week 6.

But those numbers don’t account for Hambright’s fourth-and-1 false start against the Titans. Or Coward’s early flinch against the Los Angeles Rams. That was another miscue that agitated Nagy.

“You can’t have penalties on fourth-and-1,” he said. “As hard as we’re making it on ourselves right now, you can’t have penalties. That’s the part there for all of us to figure out, ‘OK, when is this going to stop?’ ”

Given the continued struggles of a Bears offense that ranks 29th in total yardage, 32nd in rushing and 31st on third down, every slip-up becomes magnified. Quite simply, greater stakes are involved for a team that has so much difficulty scoring.

Remember the fourth-quarter toss play to Cordarrelle Patterson to the short side of the field against the Rams three weeks ago? That lost 2 yards and hurt the Bears’ comeback efforts.

What about Patterson’s loss of 1 on third-and-1 in the fourth quarter against the Indianapolis Colts?

And remember in Carolina when the Bears needed three plays to move the last 2 feet of a 56-yard touchdown drive? Remember how they first tried to run Montgomery behind five offensive linemen and four tight tends but failed, and then they tried Montgomery out of a shotgun three-receiver set and failed again? Eventually Foles ended the frustration with his 1-yard scoring plunge on third down. But the struggles involved in that sequence felt heavy.

Such difficulties have been a part of this offense’s identity since the first possession of the first game in Detroit. On the season’s third play — third-and-inches from the Bears 34 — quarterback Mitch Trubisky was too passive with a sneak attempt and got stood up, turned around and swallowed whole by the Lions defensive line. The punt team came in.

The Bears have to be much better. There’s no way around it. They have to be more disciplined. They have to be sharper with their blocks. They have to devote extra effort to picking up that 1 yard they so badly need when such situations arise.

On the bright side, Montgomery’s longest run all season — a 38-yard burst against the New Orleans Saints in Week 8 — came on third-and-1 through a huge opening. It was a rare instance in which the running back got to the second level without being touched. And then he was off and running.

Fox play-by-play announcer Joe Buck sounded shocked on the broadcast. “The Bears desperately needed that,” Buck said.

Right he was.

The Bears will need a lot more of that over the final seven games as they attempt to stop a demoralizing slide and salvage their season.