Here are five things we learned about Manchester City as their much-changed side fell to a 1-0 defeat at Old Trafford in the EFL Cup.
A Juan Mata strike was enough to send holders Manchester City tumbling out of the EFL Cup. Pep Guardiola made nine changes to Jose Mourinho's four, and the more settled side were able to see out the game and claim an important derby victory.
The loss stretched City's winless streak to six games - the worst run in Guardiola's managerial career.
Guardiola's system needs specific players to be successful
Guardiola is a world-class manager, with a world-class system. However, when paired with a set of players who do not fit into that category, the system is not likely to function as it should.
City made nine changes to the side that failed to beat Southampton at the weekend, and there was a lack of urgency and penetration as a result.
The inclusion of Nolito and bringing Raheem Sterling off the bench did not make the impact desired, as their direct, dribbling style required a player of real class in the centre to find them in dangerous areas and open up spaces.
Guardiola learned a lot about his players on Wednesday night, and some players are beginning to look as though the system has found them out. The Spanish tactician will likely be working out where the side needs strengthening; and who the next casualties of his reign at the club will be.
Vincent Kompany isn't back to his best yet
At half-time, with the game locked at 0-0, Guardiola elected to bring his captain Vincent Kompany off for Aleksandar Kolarov.
It may have been a tactical move, to bring a player on at the back who is more comfortable with the ball and better at starting attacks, but intuition says that the Belgian just is not quite fit yet, and City fans will hope there was no re-occurrence of an injury.
City have been magnificent going forward at times, but have been decidedly shaky at the back. Their club captain and best central defender would go a long way towards fixing that, but he needs to get fit, and fast.
There is a place for old-fashioned wingers, but not this time
It is known that, for all the emphasis Guardiola places on his central players, he values width very highly.
Against packed defences, the likes of Mourinho teams and Atletico Madrid in big games, the Spaniard has often utilised inferior players ahead of their more technical counterparts simply for the raw pace and width they bring to the team.
Jesus Navas is one such player. He is neither a dribbler nor a creator, but simply a pacy wide option on the right with the ability to hold width and stretch play.
He certainly has his place in a Guardiola squad, but all his endeavour was wasted against United, despite performing admirably. Luke Shaw did well in holding him up on the flank for the most part, and as a quick full-back, Navas's rather one-dimensional game was not too difficult to work out.
When the Spaniard did get the better of him, though, there were few options in the middle to pick out, with no one of real stature in the box to power home a header. One such glorious chance right at the start of the match was wasted by Kelechi Iheanacho, who fluffed his header wide.
Navas's limitations were emphasised when he switched flanks with Sterling to the left, and was unable to really affect the game from that position.
No Aguero, no goals
That title is perhaps a bit of an exaggeration - Iheanacho is a decent young striker and plenty of goals have come from midfield players - but it is no exaggeration to say that the Argentine is the most dangerous goal-threat in City's squad.
Guardiola's recent defense of his player indicates that there is no problem between player and manager, so one must assume that Sergio Aguero's fitness has been in doubt of late.
Without their main man in the penalty box, breaking down solid defences becomes that much harder.
Nolito a super-sub, not a starter?
Nolito is a talented player, but seems to drift through matches at times. When selected from the start, he is not always able to provide the intensity Guardiola demands from first whistle to last.
However, he can definitely make an impact off the bench. All three of his Premier League goals have come as a substitute. He ghosts past tired defenders and gets himself into good scoring positions.
Perhaps the combination of his age and lack of familiarity with the league are part of that, and his role as a super sub may be something for Guardiola to consider over the rest of the season.
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