It is no secret that Chelsea have not been the best at the back this season.
Their defensive frailties have been exploited time and time again, particularly since the Premier League's resumption. The recent 3-2 derby defeat to West Ham, and the narrow victory at Crystal Palace - in which they tried their very best to throw away a lead - accentuated the Blues' weakness.
On Saturday they were laid bare.
Frank Lampard’s men travelled to Bramall Lane to face Sheffield United, which is by no means an easy game. You only needed to rewind to August - if you're of a Chelsea persuasion - to realise how challenging a game it would be, when the newly promoted Blades came from behind to draw at Stamford Bridge.
Yet Lampard was in for a big surprise in south Yorkshire.
David McGoldrick struck first, reacting quicker than both of Chelsea’s centre-backs to smash past Kepa Arrizabalaga from close range. Kurt Zouma and Andreas Christensen had a simultaneous lapse in concentration, failing to react to Kepa's parry - allowing the Irishman to score his first ever Premier League goal.
Oli McBurnie added a second soon after, again a lack of awareness from the centre-back duo proving costly.
The Blades capped off the game with a third goal, which of course meant a third glaring error from Chelsea's defence. Substitute Antonio Rüdiger failed to clear his lines and put it on a plate for McGoldrick, who was on hand to smash it home for his second of the evening.
Three goals, three mistakes and three points thrown away by Chelsea.
They have now conceded nearly half a century of goals in the league this season - more than any team in the top half. The Blues have shipped eight goals in their last three away games alone, their only clean sheet on the road since the restart coming against a flat Leicester side in the FA Cup.
Simply put, Chelsea have not been up to standard defensively, and it could now cost them in the pursuit of Champions League football.
And it's clear that, while it's nice to sign Timo Werner, Hakim Ziyech and (possibly) Kai Havertz this summer, the real need for reinforcement is at the back.
If the club wants to move forward and into a trophy-laden future, they need to spend big at the back ASAP.
They have the money. They have the resources. They have the backing. There are no excuses not to dip into the market and sign defenders, though while options seem scarce at the moment, there are certainly enough to bolster the Blues’ backline.
Chelsea need to find a new left-back and a centre-back - practically half of their defence, but that is how severe the situation is. They need to go on a deep search for options, or make a move on existing targets. Leicester’s Ben Chilwell and Ajax’s Nicolás Tagliafico have been thrown into the mix, while Kalidou Koulibaly of Napoli would be an immense signing for the heart of defence.
It is not the end of the world if Chelsea do not completely overhaul their defence - after all, they still sit third with three games to play. But the club's success throughout the 21st century has been built upon a strong defensive foundation (John Terry and co.) and that's how this new-look Chelsea team should be built too.