United have been solid rather than spectacular so far this season but they were in fine form in their opening UEFA Champions League game to emphatically see of their German opponents.
Wayne Rooney scored twice to make it to 200 goals for the club, while Robin van Persie and Antonio Valencia also netted in the second half.
Fellaini played his part in a strong midfield performance on his first start for United since arriving from Everton.
"It's my first game in the Champions League so I am happy to play," the Belgian told Sky Sports.
"We won and that is good for the confidence for the next game because that is a derby and a big game for Man United.
"I play with top players - Rooney, Van Persie, Valencia, Kagawa, Carrick, so it is easy for me to play."
Smalling was equally thrilled with United's attacking display, but admitted there was still room for improvement defensively.
"We all felt it was a good team performance overall," said Smalling, who lined up at right-back.
"We felt that we dominated the first half and to be honest we didn't really want it to end, so we needed to make sure it didn't drop and I felt we really stepped it up and got the goals."
Asked about the goals Leverkusen scored, Smalling said: "They were very sloppy goals from our point of view considering they didn't create many chances, but we didn't let our heads drop when they scored and we went up the other end and made them pay.
"We had really good movement in terms of Antonio, Wazza and Robin really running at them and that put them on the back foot."
Smalling also declared that he was relishing the competition for places under David Moyes, adding: "I've been training really hard and whether it's right-back or centre-back, I'm ready."