Manchester United entered this afternoon's derby against Manchester City having enjoyed a stellar start to the new Premier League season.
One of the key players in the Red Devils' hot start has been Daley Blind, who's performances at centre-back have kept Chris Smalling out of the starting XI.
All that good work may have just been undone, however, after the Dutchman was at fault as Kevin De Bruyne fired City into the lead at Old Trafford in the first half. As you might expect, the reaction from United fans and others on social media wasn't exactly sympathetic.
Perhaps Blind was surprised that City deployed something of a route one method to get De Bruyne through on goal, or maybe his lack of anticipation as Kelechi Iheanacho flicked on a long ball was evidence that he isn't a natural fit at the centre of defence.
Whatever the case may be, fans were clear on one thing - United falling behind early on lays firmly on the shoulders of the Dutch international.
Here's some of the best reaction...
Few could blame Mourinho for starting Blind ahead of Smalling given his form so far this season.
However, his lack of composure and failure to handle a City front line missing Sergio Aguero means the calls for Smalling's return to the line up are intensifying...
With Man City having gone on to take a 2-0 lead - thanks to Blind inadvertently playing Iheanacho onside - it surely won't be long before Jose Mourinho makes some changes.
Whether he is bold enough to haul off one of his centre-backs when United's attack has also struggled remains to be seen.