The Reds’ European campaign got off to an impressive start on Tuesday as Wayne Rooney and Robin van Persie both scored again to help defeat the Bundesliga outfit 4-2, while Antonio Valencia got on the scoresheet too.
"The victory has boosted our confidence for Sunday," De Gea told ManUtd.com. "It was important to kick off with a win; we made a good start and got the three points, which gives us peace of mind.
"We’ve since had a few days to prepare for City and we’re looking forward to the match."
David was full of praise for Rooney, who scored twice against Leverkusen to become only the fourth man to reach 200 goals for United.
"Not only did he score goals, he played some great football, worked hard, brought dynamism to the team and played a really great game," the goalkeeper said.
"It’s good for us that he and Robin score goals. It is important; they are key players and this year they will combine to great effect."