Mats Hummels said Germany's tactical approach was wrong after the World Cup holders lost the opening game of their title defence 1-0 to Mexico.
Hirving Lozano struck the only goal of the Group F clash, but Mexico wasted numerous chances on the break to record an even larger margin of victory.
Germany, meanwhile, had 26 shots without causing Guillermo Ochoa too many problems, although the Mexico goalkeeper made nine saves in keeping a memorable clean sheet.
Hummels felt Germany did not heed the warning of their final warm-up friendly against Saudi Arabia that ended in a tight 2-1 win, their only success in seven matches since the end of the World Cup qualifying campaign last year.
"It's pretty simple. We played like we did against Saudi Arabia, but against a better opponent," Hummels said to ZDF.
"We talked about a few things, such as not losing possession and protection, which unfortunately we did not put into action today.
"Mexico deserved the game because we made it too easy for them, knowing we should not have allowed that to happen.
"If seven or eight players play offensively, then it's clear that the offensive force is greater than the defensive stability. That's what I often say internally.
"Our stability is not good, you have to say - often only Jerome [Boateng] and I were at the back, so they mercilessly attacked us."
Germany will still expect to progress - they face Sweden and South Korea in their remaining matches - but Hummels told his team-mates to buck up their ideas.
"A wake-up call is too late, we now have to win two games, otherwise the World Cup is over," Hummels said. "We actually had the wake-up call against Saudi Arabia."
1 - Opening game at the World Cup for reigning champions in the 21st century:— OptaJean (@OptaJean) June 17, 2018
2002 France 0-1
2006 Brasil 1-0
2010 Italy 1-1
2014 Spain 1-5
2018 Germany 0-1
Pressure. #GERMEX #WorldCup pic.twitter.com/8g3kXZHSwA
Real Madrid midfielder Toni Kroos joined Hummels in demanding an improved performance when Germany take on South Korea on Wednesday in Kazan.
"I think we did not find any solutions in the first half," Kroos said.
"Mexico were clever, we did better in the second half and Mexico got tired.
"We're under pressure now, without question, we have to get six points from the two games, if possible."
16 de julio 2018
|Familiarity with demands of Real Madrid an advantage – Lopetegui|
|Parmentier rallies to avoid Bucharest exit, Arruabarrena bows out in Gstaad|
|Steelers fail to sign Bell to long-term deal|
|Over 10 per cent of Croatia´s population welcomes World Cup heroes home|
|Thierry Henry quits pundit role to focus on coaching|
|France´s World Cup winners given heroes´ welcome in Paris|
|At the end we will see who is best - Ronaldo sidesteps Messi rivalry|
|Qatar? China? Ronaldo only has eyes for glory at Juventus|
|Proud Kane praises team-mates after Golden Boot win|
|The Open 2018 diary: Is that Tiger Woods? Harrington feels the Burn...|
|Porte targeting Vuelta after Tour de France anguish|
|Maradona madness, #WaistcoatWednesday, bruised Batshuayi - Alternative World Cup awards|
|It could be the best time - Martinez hints at Hazard Chelsea exit|
|England call up Billings as cover for injury doubt Roy|
|McIlroy on track for fitting major haul - Harrington|
|Froome plays down Thomas battle for Sky supremacy|
|Carnoustie record counts for little at The Open, says Fleetwood|
|I don´t see it - Napoli chief De Laurentiis plays down talk of Callejon-Suso swap|
|One or two changes required at Chelsea, says Sarri|
|Monaco confirm Golovin approach|
|Let´s keep making noise! - Serena dedicates Wimbledon run to fellow mothers|
|Full list of Open tee times|
|Fakhar inspires another big Pakistan ODI win|
|Spieth ´annoyed´ by re-watch of 13th hole|
|Kaka claims Ronaldo arrival will boost all Italian clubs|
|Spieth vows to be more artist than tactician in Open defence|
|Conte thanks Chelsea players, staff and fans|
|Rodgers: I´d love to play with the Packers to 40|
|Sevilla insist N´Zonzi will cost €40m|
|Hugill shows mystic powers with accurate World Cup predictions|
|World Cup 2018: FARE ready for post-tournament challenges in Russia|
|Woods grouped with Matsuyama and Knox at Open|
|Matthaus questions idea of French dynasty|
|Chandimal, Sri Lanka coach and manager banned for two Tests and four ODIs|
|Sassuolo demand at least €30m for Berardi|
|French media revels in ´crazy´ triumph|
|Mafi released on bail after alleged assault|
|Bierhoff must go! Berthold calls for Germany changes|
|Joshua to fight Povetkin in September|
|World Cup 2018: Goal-laden final adds to staggering World Cup stats|
|Runners and Ryders: Molinari impresses again as stars prepare for The Open|
|Macron to greet France team after Champs Elysees parade|
|Casillas and Suarez critical of VAR after World Cup final|
|TMO protocol clearly not working, says SANZAAR|
|Americans seek joy of six at Carnoustie, Rose out to end wretched run - The Open in Opta facts|
|Ronaldo arrives in Turin for Juventus medical|
|Stone: I had no idea what my score was!|
|The Open: A look back at Van de Velde´s Carnoustie calamity|
|Rakitic proud of ´brother´ Modric´s Golden Ball award|
|Serena can surpass Court, but Graf was the real target - Krajicek|
|Ronaldo moved to remain motivated - Kaka|
|Silva confident on Everton transfer targets|
|Golden Boot winner Kane aims to break August hoodoo|
|NFL training camp 2018: Contract holdouts and where they stand|
|Guardiola took me to another level - Alaba|
|Madrid selling Ronaldo a ´historic error´ - Calderon|
|MLS Review: LAFC, Atlanta held at home|
|Stones will watch re-runs of England´s World Cup matches|
|Ferdinand challenges Mourinho to unleash Pogba at United|
|Mbappe: I´m staying at PSG|
|Former NHL goalie Ray Emery found dead|
|Pirates secure five-game sweep of Brewers with walk-off win|
|WATCH: LeBron receives standing ovation at Lakers´ summer league game|
|Tony Romo wins celebrity-filled American Century Championship|
|McGregor praises Putin after attending World Cup final|
|Open-bound Kim claims maiden title with eight-shot triumph|
|Eagles running back Sproles plans to retire after next season|
|World Cup winner Rami retires from international football|
15 de julio 2018
|Wimbledon champion Djokovic did not expect grand slam glory so soon|
|Ibrahimovic praises Pogba after World Cup final display|