It was almost a eureka moment.
"That's why Jose Mourinho sold Juan Mata to Manchester United," declared Jamie Carragher.
It may have been David Silva who was the subject of his criticism, but the gap left by the Spaniard, which contributed to Branislav Ivanovic scoring the game's only goal, explained as much about Chelsea's recent transfer dealings as it did City's weaknesses.
Of course, Mourinho had known it all along.
David Silva and Juan Mata are different players, yet similarities remain in their approach. The pair prefer to float between the lines, drifting into space to receive possession and unlock defences.
Their strengths are very much when their team is on the front foot. Defensively, they offer little.
Indeed, a man of Mata's talents would have probably proven the difference in Chelsea's recent goalless draw against West Ham United.
The Blues struggled to break down a stubborn Hammers rearguard at Stamford Bridge and, although they recorded 39 shots on goal in the 90 minutes—a Premier League record for a team not finding the back of the net—they couldn't find a way through.
Chelsea needed Mata that night, only he had been sold for £37.1 million to Manchester United a week earlier.
It was a different scenario against City. The stakes were higher and, for all his talents, Mata's defensive shortcomings would have far outweighed what he offered Chelsea going forward.
The Blues' victory was one built on tactical discipline, with all 11 players in the Chelsea line-up perfectly executing Mourinho's meticulous plan for victory.
The plan didn't include a player of Mata's ilk. Fielding David Silva, Manuel Pellegrini's plan did, of course, and it was the Chilean manager who saw his team suffer defeat.
Ivanovic's goal on Monday came from a Manchester City attack, a long ball pitting Eden Hazard against Martin Demichelis.
As he did for much of the game, Hazard evaded the Argentine's attentions, squaring to Ramires. As play continued, Hazard soon found his way into space in the box, receiving possession from Ivanovic before picking out Ramires once more.
Vincent Kompany bravely blocked the Chelsea midfielder's effort, the deflection landing in the path of Ivanovic, who did the rest with a sweetly struck shot that flew past Joe Hart in goal.
A wonderful breakaway goal.
As fluid as the Blues appeared, City appeared all at sea in contrast. Their defensive midfielders Yaya Toure and Demichelis had been pulled out of position, leaving gaps in dangerous areas, but it was David Silva who ultimately proved the weakness.
Despite the threat of Chelsea on the counter, David Silva had failed to track back and pick up his man, Ivanovic. It meant City were exposed, and when the ball eventually fell to the Serbian right-back, he knew he had enough time and space to unleash his killer blow.
"I don't think Mourinho would entertain a player like David Silva," Gary Neville added to Carragher's analysis on Monday Night Football.
Will selling Juan Mata come back to haunt Chelsea?
Both pundits were right; Mourinho wouldn't. It's why he allowed Mata to leave.
Games like Chelsea's West Ham draw will come along for any team over the course of a season. As frustrating as it is, those dropped points are retrievable, though.
When it comes against your rivals, however, it's a far more difficult task to claw back lost ground.
Without Mata, Chelsea have Oscar, Andre Schurrle, Mohamed Salah, Willian and Hazard in attacking midfield. They're dynamic, athletic and capable of grafting defensively as much as they are talented in causing havoc further forward.
The latter pair in that list were vital against City, operating in a way their former teammate cannot.
It's why the Blues were able to complete a Premier League double over City on Monday. It's why Mata may be missed as a character in the Chelsea dressing room, but on the field it's an altogether different tale.
Garry Hayes is Bleacher Report's lead Chelsea correspondent and will be following the club from a London base throughout the 2013-14 season. Follow him on Twitter here @garryhayes