Armed with a 3-1 lead from the first leg at Old Trafford, City approachea the return fixture in the semi-final as firm favourite to reach the 1 March showpiece at Wembley.
However, City know all too well how potent United can be on its day, having suffered a home league defeat to Ole Gunnar Solskjaer's men in December.
Guardiola vowed he and his team have never been complacent, and he is optimistic that attitude will be enough to deliver an aggregate success.
"It's Man United. They can do it and have done it," Guardiola said. "In this kind of game, against this kind of team, with this incredible history, they have pride and I know, I know how difficult it would be to reach the final."
Speaking about the need to concentrate against neighbours who have endured a difficult Premier League campaign so far, Guardiola indicated he trusted his players to bring the right application.
"Never was my team complacent during four years - never," he said. "We have to be careful about their skills, we have to be careful about the counter-attack, we have to be careful about the set-pieces, we have to be careful about how good they can play if we let them play.
"We will have to defend. They don't have anything to lose so they are going to attack. We will have to control our emotions. We have to try to score goals."
Asked about possible squad rotation, a fired-up Guardiola stressed managers should be allowed to select their teams as they feel appropriate.
That issue has picked up traction in the wake of Liverpool boss Jurgen Klopp saying it would be the club's under-23 side that face Shrewsbury in their FA Cup fourth-round replay at Anfield, given the first team will be taking their mid-season break.
"Listen," Guardiola began. "We accept the schedules, the competitions, the games every two or three days, but don't tell the managers which selections you have to choose, please. It's our job."
City has a doubt over centre-back Aymeric Laporte but is expected to field a strong side, being eager to give United no hint of weakness.
City could have won by a greater margin at Old Trafford in the first leg, when Red Devils boss Solskjaer said City's strong line-up was a sign of its "respect" for United.
"I show respect for my opponents from day one, from when I started with the second team at Barcelona to today, sat in front of you here," Guardiola said. "I took that selection (at Old Trafford) and tomorrow I will take another selection."