Manchester City lost their first Premier League game of the season over the weekend, losing 2-0 to Chelsea, as Liverpool leapfrogged them to the summit of the table.
While the result should cause little worry for the Citizens long-term considering the quality of some of their play, especially in the first half, concerns regarding Kyle Walker's defensive capabilities reared their head once again.
The 28-year-old was singled out for the Blues' first goal, as the full-back allowed Willian to get in behind him in the buildup to N'Golo Kante's opener, which perhaps still highlights a persistent fault in his game.
Rio Ferdinand, albeit off-air, labelled Walker 'a liability at the top level' during his punditry for BT Sport, which is where the majority of criticism stems for him, that for all of his pace and attacking endeavour, the defender can switch off in the big games for both club and country.
Walker is seemingly the perfect full-back in a Guardiola team, offering energy and dynamism down the flanks, similarly to that of Dani Alves during his time at Barcelona. However the defender is yet to supply an assist this season, scoring only once, compared to his record of five assists at this stage last season, which is usually the argument for having him.
This isn't just an issue exclusive to club level, as England found out during the World Cup over the summer. Playing as a right sided centre back, the City man was caught under the ball from a cross during the Tunisia opener, resulting in the defender giving away a penalty due to his poor positioning that almost cost the Three Lions the win in their opening group game.
It would prove to be a sign of things to come for Walker, as his poor positioning in the semi-final against Croatia meant he couldn't see an onrushing Mario Mandzukic coming across him at the far post to equalise, before the Croats went on to win in extra-time.
Walker has become a key player for both club and country, with very few full-backs able to offer the raw physical attributes he possesses, however the Chelsea game once again shed light on a persistent weakness in his game; something that can't simply be rectified by sheer pace.
City are currently facing a genuine challenge in the shape of Liverpool as they look to retain the Premier League title, with the club still harbouring ambitions of winning the Champions League, while England have the Nations League finals to prepare for in 2019, meaning defensive lapses from Walker during big games must become a thing of the past, and quickly.