We were actually very well set up to defend the corner they scored from, (I only logged in here yesterday since the NLD as I thought it best avoid the usual lunatics in here, so I bet the corner has been done to death with Romero taking all the blame accused by people that have never defended a corner in their life no doubt too) BUT very quickly I want to note that the failure to defend their corner was the fault of Benancur and Solonke who were their marking zonally, the zone in which the ball was aimed at for Gabriel to run into, BOTH of them got under the ball and didn't clear it. (It's almost impossible to man-mark a running into the box on corners as you are always reacting and therefore always never positioned correctly).
As you say City were really bad shape, they tried to do a similar thing to us half the team zonal the other half man mark (I think the man marking is for our bigger guys to man mark their big guys??) but City's zonal players are nowhere to be seen because they get sucked into the back post runners of the scum, it left a huge hole at the back post on the goal line. Our zonal players remained in their zones, they just fucked up the clearance.