The Toffees lost 1-0 at home to United on Saturday, following a first-half goal by Javier Hernandez.
“We’re disappointed, it’s always a difficult game against Manchester United but we had three or four chances to score but we couldn’t,” the 23-year-old told the club’s official website.
“Manchester United only made three or four chances but scored. It’s not easy against a big team and we played well but couldn’t score.
“If you don’t score you can’t win and when that happens you are less confident, but hopefully next Saturday we can win.”
Fellaini will miss Everton’s clash with Newcastle after picking up a fifth yellow card of the season.
“It’s difficult, I haven’t missed any games this season and I don’t want to miss a week,” he added.
“But hopefully I’ll come back from international duty feeling fit.”
