Playing a high-octane pressing game to try to force City into making mistakes, Bournemouth took a deserved lead seven minutes later when they won the ball in the opposition half and striker Evanilson was on hand to bundle it over the line from close range.
Guardiola introduced 20-year-old O'Reilly at the break and he quickly created the chance for Haaland to redeem himself, playing the ball across the face of goal for the Norwegian to slam home at the far post in the 49th minute.
Shortly afterwards Haaland suffered the ankle injury that effectively ended his afternoon, but Marmoush made light of his absence, slotting the ball in two minutes after coming on to secure City's spot in the last four of the competition, where they will meet Nottingham Forest on the weekend of April 26.
Guardiola said that O'Reilly, who set up both goals, would definitely play in the semifinal, and the young midfielder said he was delighted to be able to contribute, despite playing in an unfamiliar position.
“I am just trying my hardest every game and trying to help the team out as much as I can and I'm glad I've been able to do that so far,” O'Reilly said.