"But they're very rarely exposed, you don't get chances against them that much".
Mourinho claims the pressure he is under is the "fault" of United's legendary past success.
"There are little things in his game that I am not a big fan. When I say fault, I mean for the right reasons", he said.
"At the moment, he has Lindelof and Smalling but they don't inspire confidence throughout the team and as soon as that transfer window opens I'm sure they will be out looking for centre-backs again".
In the Spaniard's first season, the club recorded a 0-0 draw with United, while the Red Devils came back two goals down to record a memorable 3-2 win in last season's encounter at the Etihad.
Mike Pompeo delays talks with senior North Koreans
North Korea's Foreign Ministry said the US President Donald Trump's administration had failed to understand its repeated demand. Pompeo was supposed to travel to NY on Thursday to meet with his North Korean counterpart, Kim Yong Chol.
'Is scoring goals in one career, is scoring goals [in] like the Spanish league'.
Asked if United, who have won their last three games with a more attacking approach, will go toe-to-toe with City on Sunday Mourinho said: "We are going to try".
Mourinho was quick to pay tribute to Aguero's qualities as a goalscorer before United's latest clash with arch-rivals City, but he dropped a strong hint that he believes there is a dark side to his game.
"So if we keep conceding goals before the opponent we will arrive at the day where we can not come back so we have to make sure that we grow up, not just in our approach, but also in our compactness in order to start matches well".
"Lukaku is training with the team this morning (Friday)".