This season has been a mixed one for Manchester City. Going forward, they have been as exhilarating as ever and perhaps even better. However, when it comes to their defence, things have not been so rosy.
With City one man lighter after losing Vincent Kompany to Anderlecht, Aymeric Laporte picked up a knee injury which is expected to sideline him for the majority of the campaign, and now John Stones is poised to miss five or six weeks of action with a muscle injury.
City are left with just one senior centre-back, Nicolas Otamendi, who has been Guardiola's clear third-choice option since his arrival. The Argentine is now set for a long run in the team, but who will join him?
On paper, it looks bad for the Citizens, who are set to take on Shakhtar Donetsk in the Champions League on Wednesday. However, things aren't as bad as they may seem.
In their current senior squad, they have plenty of options to get around this crisis. The leading candidate to fill in is Fernandinho, who has already been deployed as a centre-back this season. The Brazilian boasts the defensive awareness to keep his former employers at bay, and is good enough on the ball to help keep City moving forwards.
Whilst he would be a big loss in midfield, the summer signing of Rodri should help. The young Spaniard is seen as the heir to Fernandinho's throne, so it could be time for him to step up now.
Should Guardiola prefer to keep Fernandinho in midfield, he could turn to Kyle Walker. The right-back has some experience centrally after playing there for both City and England, although that was often as part of a back three.
Walker, who could be replaced by Joao Cancelo on the right, perhaps lacks the defensive awareness to cope as part of a traditional partnership, so Guardiola may again decide against doing this.
The next option would be to turn to the academy. Both Taylor Harwood-Bellis and Eric Garcia were named in City's Champions League squad, and Garcia has actually travelled with the team for the game.
The 18-year-old Spaniard has been tipped for big things, and Guardiola is well aware of him after handing him three Carabao Cup appearances last season. He has never played in a game of this magnitude, but Garcia may be set for an unexpected European debut.
Whilst none of these options are exactly ideal for Guardiola, he doesn't really have much choice. City decided against recruiting a new centre-back this summer, and they are now paying for that choice.
Nevertheless, the overall quality in the squad should see them through. There are plenty of competent defenders who could get the job done, and City will still see themselves as overwhelming favourites for the game.