In the immediate aftermath of his team's season-opening defeat to the Los Angeles Chargers at FedExField, Washington coach Ron Rivera felt reasonably upbeat about his team's performance. Sure, the coach had been frustrated by his defense's inability to get the ball back after a punt from Chargers territory with 6:52 remaining, but all in all the 20-16 setback to an emerging team and its radiant young quarterback didn't seem so heinous.

"One less mistake -- that's the difference in this football game," Rivera told his players as they gathered in the locker room Sunday afternoon. "One less mistake, alright? That's the truth of the matter."

Twenty four hours later, having turned on the game tape and endured what amounted to a coach's equivalent of a horror flick, Rivera concluded that he had been mistaken.

"I was a little disappointed after the game, but I'm really disappointed after watching the tape," Rivera said Monday following the team's walkthrough practice, part of a compressed preparation program for Thursday night's nationally televised home game against the New York Giants. "What stuck out was the lack of discipline. It wasn't just a couple of mistakes; there were more than enough to get upset about. And it goes back to everything I've been saying about the lack of maturity."

Early in training camp, Rivera shared his biggest fear about Washington's 2021 team: That in the wake of a surprising late-season charge to the NFC East title, and a stirring playoff performance against the eventual Super Bowl champion Tampa Bay Buccaneers, his players -- especially the younger and less experienced ones -- would head into this campaign with a false sense of security. If players believed they could flip a switch and pick up where they left off last January, rather than remembering the collective grind and commitment it took to get there in the first place, a reality check could be swift and severe.

"You're not dropped on top of the mountain," Rivera reminded his players.

In Week 1, Washington face-planted. For all the concern about instant upheaval at the sport's marquee position, with newly installed starting quarterback Ryan Fitzpatrick suffering a hip injury that knocked him out of the game in the second quarter and may keep him out until after the leaves turn, it was the team's vaunted defense that disappointed most glaringly down the stretch.

When he turned on the tape Monday morning, Rivera found one play particularly hard to stomach: With the Chargers facing third-and-16 from their own 12 with 5:29 remaining, second-year quarterback Justin Herbert dropped back and zipped a ball over the middle to star wide receiver Keenan Allen, who collected it and fell backward for a 17-yard gain. Allen got to the first-down line despite the presence of three nearby defensive backs (safeties Bobby McCain and Landon Collins and cornerback Kendall Fuller), while Herbert had a completely clean pocket -- none of which pleased Rivera.

"We could have benefitted from some pressure," he said. "And we could've gotten a little bit better coverage and forced that ball to be thrown underneath, and not over the top (of the linebackers)."

That was merely the first of Herbert's four aerial third-down conversions on the game-clinching drive; all in all, the 14 third-down conversions allowed by Washington were tied for the most in franchise history.

In other words, there was plenty of blame to go around.

While Rivera was less unhappy with the offensive performance, which included an efficient and reasonably clean effort from Taylor Heinicke in relief of Fitzpatrick, the game tape revealed some mystifying missed blocks -- and he was obviously stung by running back Antonio Gibson's fourth-quarter fumble in the shadow of his own goal line, setting up what proved to be a game-winning touchdown drive that totaled a mere three yards.

Eliminating mistakes completely isn't a realistic goal, but Rivera would like to see some quick and immediate improvement when it comes to reducing the types of errors he perceives as easily avoidable. Mental mistakes raise his heart rate. Players freelancing, even when their intentions are good, make his blood boil.

"Sometimes it's guys trying too hard to make plays," Rivera said. "And in the process, they're not maintaining their gap integrity. I talk to the team about playing their role, about all 11 guys doing what they're supposed to do. If one or two guys don't do their job, the opponent will find you, and exploit you.

"That's the maturity part, and that's the disappointment. Because guys paying attention to detail and doing what they're supposed to do -- that's exactly what got us to where we were at the end of last year."

A year ago, Washington fell to 2-7 before turning its season around. Included in that rough stretch were a pair of close defeats to the division-rival Giants which underscored the importance of playing clean.

In the first meeting, two Kyle Allen turnovers hurt Washington in a 20-19 heartbreaker -- Rivera, after Cam Sims caught a touchdown pass with 36 seconds remaining, chose to put it all on the line with a two-point conversion, which failed after Allen scrambled and failed to find a receiver.

Three weeks later, Washington had five giveaways (including three Alex Smith interceptions), to none for the Giants, and fell into a 20-3 halftime hole before rallying to lose 23-20.

"Turnovers are what got us, in both those games," Rivera said. "We have to protect the ball, and we have to take advantage of our opportunities. And there will be opportunities."

Rivera certainly is aware that ball security is not Daniel Jones' forte. The Giants' quarterback has lost 18 fumbles in 28 career games, and Rivera hopes his defenders can pad that stat without overextending or blowing assignments in the process.

As frustrated as the coach was on Monday, he was also excited by the idea of a quick turnaround against a division rival.

"The mistakes we made [against the Chargers], it's one of those things you can't change, but you can correct," he said. "I believe this is an opportunity to get past it very quickly and start focusing on someone else."

That's a message Rivera began driving home a few minutes after Sunday's disappointing defeat, telling his players in the locker room, "This one's tough, alright? It's gonna stick in your f----- craw. Who we are, alright, is not defined by this game. How we react to it will define who we are."

That, to Rivera, is the truth of the matter.