The loss saw the Red Devils remain in second place in the table but they are now five points adrift of leaders and neighbours City and are level on points with third-placed Tottenham.
This was always likely to be the crucial phase of United's early season run, with Liverpool game quickly followed by next Saturday's game against Tottenham and a clash with Chelsea a week later. That made United's upcoming campaign particularly interesting. Goals from Aaron Mooy and Laurent Depoitre inspired the 2-1 win and Mourinho tore into his players following the final whistle, criticising their attitude and application.
The United goal prompted a good response from Swansea and the home side were nearly gifted a goal when Daley Blind headed the ball against the thigh of Oliver McBurnie, which saw it ricochet towards goal but bounce a few yards wide.
Swansea continued to push for an equaliser as half time approached and Sam Clucas nearly squeezed a curling shot into the bottom corner but the ball trickled just wide of the post.
Specifically, it's a fourth round meeting between Swansea City FC and the red-favouring team from Manchester known, of course, as Manchester United. The tempo of the game was incredibly low and a number of fouls broke up play to such an extent that genuine attacking play became a rarity.
"Scott McTominay did very well, strong in the midfield".
He placed a header from the edge of the box perfectly to convert a Matteo Darmian cross.
Sergio Romero made late stops from substitutes Tammy Abraham and Leroy Fer, but United eased into the last eight after sending on Nemanja Matic and Romelu Lukaku from the bench.
"But I don't think [it was easy] I think they have a good squad, a squad with lots of options".
It's the League Cup!
Manchester United's players reportedly believe that Jose Mourinho went too far with a tantrum after they lost 2-1 to Huddersfield over the weekend.
"Since the first minute we felt we were in the game, we had immediately some corners, pressing high and comfortable on the pitch".