John Stones has shown remarkable improvement in his second season at Manchester City, and his absence is a real dagger for Pep Guardiola's unbeaten side.
This time last year, there would have been some Manchester City fans pleased to see £50 million signing John Stones out of the first team. Not out with an injury of course, but just away from the starting lineup and away from the distracting scrutiny.
The defender seriously struggled in his transition to a big club, and it was clear to see that the pressure of the price tag was weighing heavy on his young shoulders.
Fast forward to November 2017, and things have picked up significantly for the 23-year-old, as supporters had so desperately hoped they would. Stones has been a pillar of confidence at the back for City in front of new signing Ederson and has been, arguably, the best center half in the Premier League this season so far.
The Barnsley-born star has added defensive resolve to his game to accompany his highly impressive ball-playing style and now all of sudden, there could be a problem for City as they head into the busy festive period.
It has been confirmed that the hamstring injury he picked up after half an hour against Leicester on Saturday will result in a lay-off of around four to six weeks, meaning he could miss approximately 11 matches for the Citizens.
It will mean that City now have to hope Vincent Kompany remains fit and with a cluster of games coming thick and fast, that will be a test for the injury-prone Belgian skipper, who has only just returned from a two-month absence.
Kompany has been without doubt one of the finest center-halves to have ever graced the Premier League, but at the age of 32 and having been on the sidelines for so long in recent years, it's not easy to say that he will just slot straight back in and do the job - he looked suspect in parts against the Foxes and was almost sent off within a minute for taking down Jamie Vardy.
Nicolas Otamendi has been in great form alongside Stones but how will he react to playing alongside someone else on a matchday? The pair struck up a very strong partnership but now the Argentine is going to have to adapt and understand - certainly if he plays with Kompany, who won't have even been training alongside Otamendi.
Then there's Eliaquim Mangala, who replaced Stones as he hobbled out at the King Power Stadium. The Frenchman hasn't really cut the mustard since joining the club in 2014 from Porto, and he's nowhere near as composed on the ball as Stones, which could be detrimental to City's phenomenal build-up play from the back.
City can afford to lose players like Kevin De Bruyne, Sergio Aguero, David Silva and Leroy Sane, because there's so much strength in depth in the attacking areas. With stars such as Ilkay Gundogan, Raheem Sterling, Bernardo Silva, Yaya Toure and Gabriel Jesus waiting in the wings ready to be called upon, the blow of any niggling injuries to starters can be eased.
But Stones is one of only a couple of players in the side at the moment who would be a genuinely massive miss for City - the other being the irrepressible Fernandinho and the qualities he brings to that holding midfield role. Make no mistake about it, the club is now facing a far tougher prospect of remaining unbeaten and could be about to ship goals at a higher rate.