Manchester City are enduring their worst slump in form since Pep Guardiola took charge as manager in 2016. Last season's Treble winners are on a four-game winless streak in the Premier League and have dropped to fourth in the table, six points behind leaders Arsenal, after Wednesday's 1-0 defeat at Aston Villa.

City have conceded nine goals during that sequence of results and against Villa in midweek, Guardiola's side allowed Unai Emery's team 22 shots on goal (City managed just two), which was the joint-most faced by a Guardiola side in a game within Europe's big-five leagues -- a run of 535 games -- according to Opta.

The champions travel to Luton Town on Sunday aiming to end their winless run and claim a league win for the first time since a 6-1 home win against Bournemouth over a month ago, but the trip to Kenilworth Road has now become an unexpected test of City's title credentials.

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Guardiola's team remain capable of bouncing back and repeating last season's Treble, so why are they in such a poor run of form and can City overcome the issues that have sent them veering off course in the Premier League?

Injuries

Every team suffers from injuries. Tottenham, Manchester United and Liverpool have all been without key players for significant periods this season, so Manchester City have not been uniquely troubled by injury problems. Yet the absence of Kevin De Bruyne since the opening game of the season with a serious hamstring injury has denied City their captain and best midfielder. He is the heartbeat of Guardiola's team and the player who creates so many chances for Erling Haaland.

John Stones, a big performer in central defence and midfielder, has only started four Premier League games so far, while Jack Grealish, Mateo Kovacic, Matheus Nunes and Nathan Aké have also missed a number of games due to injury.

Against Villa in midweek, Guardiola wasn't even able to name nine players to fill the substitutes' bench, despite the depth of their expensively assembled squad.

No Rodri, no points

Is Rodri really Manchester City's most important player -- even more important than striker Erling Haaland or goalkeeper Éderson?

The Spain midfielder was a central figure in last season's Treble success, scoring the only goal of the game to win the Champions League final against Inter Milan, but his true value to City has only become evident with the games that he has missed this season.

Rodri has missed four games due to suspension - three because of a red card, one due to accumulation of bookings - and City have lost all of them without the former Atletico Madrid player. Newcastle eliminated City from the Carabao Cup in September at the start of Rodri's initial three-game ban before Wolves and Arsenal considered Guardiola's side to successive league defeats. And with Rodri missing through suspension again on Wednesday, Aston Villa capitalised on his absence by over-running City and midfield and claiming a deserved 1-0 win.

Kovacic, Nunes, Rico Lewis and Kalvin Phillips have all deputised for Rodri, but none of them are as effective as shielding the defence and controlling the tempo of game as Rodri.

Opponents being bolder

City struck fear into their opponents last season, and it's not hard to see why. They hit six past Manchester United in one game, while Arsenal, Liverpool, Spurs and Chelsea all conceded four in games against Guardiola's team last term. Even Real Madrid slumped to a 4-0 defeat against City at the Etihad in the Champions League.

This time around, opponents have been more prepared to attack City. Perhaps they have decided that attempting to contain Guardiola's side is futile, so why not try to land some blows instead? Chelsea put four past City at Stamford Bridge and Spurs scored three at the Etihad recently. Arsenal ended a 12-game losing streak against City, while Liverpool halted their 23-game winning running at the Etihad last month.

But it's not just the so-called Big Six who are being bolder against City. Wolves shocked them at Molineux with a 2-1 win earlier this season, while Villa beat City for the first time since 2013 in midweek.

Defensively, City have only conceded two more goals (17) after 15 league games than they did last season (15), but nine of those goals have been shipped in the last four games, against Chelsea, Liverpool, Spurs and Villa. So their top four rivals are clearly benefiting from being more adventurous against City this season.

Left-back issues

City don't have a reliable left-back, and it could even be argued that Guardiola's squad doesn't actually possess a specialist in that position. Since the summer window of 2022, City have parted company with Oleksandar Zinchenko, João Cancelo and Benjamin Mendy, but haven't replaced any of them. Aymeric Laporte, a left-sided centre-half who filled in at left-back at times, has also moved on.

Nathan Ake has played as left-back and done a reasonable job there, but the Netherlands international is really a centre-half. Summer signing Josko Gvardiol has been deployed at left-back for much of this season, but like Ake, the Croatia international has forged his reputation in the centre of defence.

On the right, City boast one of the world's best right-backs in Kyle Walker, but their left flank is a weak link and opponents are targeting that position.

Poor summer transfers

City have been the masters of the transfer window in recent years, with the 52 million signing of Erling Haaland from Borussia Dortmund in summer 2022 arguably the best business done by any club in the Premier League. But Guardiola's squad was arguably weaker at the start of this season than it was at the end of last due to the comings and goings at the Etihad.

City failed to persuade Ilkay Gündogan to sign a new contract, and the midfielder who was so important last season chose instead to move to Barcelona. Mateo Kovacic and Matheus Nunes were signed to help plug the gap, but Gundogan has proven to be irreplaceable.

Jérémy Doku has done well since being signed as a replacement for Riyad Mahrez, but Gvardiol has yet to settle in following his 77 million move from RB Leipzig. Gvardiol is a top defender, but City's priority should have been a new left-back rather than another centre-half.

And every time Cole Palmer plays for Chelsea, City's decision to offload the young forward looks increasingly difficult to comprehend.

Treble hangover?

Repeating last season's incredible success of winning the Champions League, Premier League and FA Cup was always going to be a tough task -- trebles are rare because they are so hard to achieve -- but maybe we're now seeing how much it took out of Guardiola's players physically and emotionally. Some of last season's top performers (Rúben Dias, Jack Grealish, Bernardo Silva) have been unusually inconsistent this time around, while De Bruyne has once again been sidelined by the hamstring problem that troubled him for the final months of last season.

Dias has made mistakes in defence and Grealish has lost his spark, registering just one goal and one assist so far in the league. Bernardo, meanwhile, has had highs and lows and is struggling for consistency.

Are things about to get better?

Perhaps this blip is temporary, City have simply suffered from a tough run of fixtures and are now about to rediscover their form and surge back to the top of the table. Having failed to win any of their last four games against Chelsea, Liverpool, Spurs or Villa, City's next four league games look far more manageable and could see them bank 12 points from 12.

Promoted side Luton Town away is this Sunday, followed by Crystal Palace (at home), Everton (away) and Sheffield United (home) before the end of December. Sandwiched between the Palace and Everton games is a trip to Saudi Arabia for the FIFA Club World Cup, meaning City could get to the end of 2023 on the back of a four-game winning streak in the league having just been crowned world champions.

That's the positive outlook. Recent form suggests otherwise, but if any team can emerge from a slump and turn the situation around, it's Pep Guardiola's Manchester City.

The difference this season, though, is that City have plenty of rivals capable of halting their march to silverware.