Fellaini jokingly said he knew Anthony Martial would miss after scoring at West Ham.
Man Utd bounced back to winning ways.
Marouane Fellaini scored as Manchester United beat West Ham to reach the semi-finals of the FA Cup, and the big Belgian said scoring a goal is good for confidence.
The Red Devils were brilliant at Upton Park on Wednesday evening as they defeated the Hammers 2-1 in the FA Cup quarter-final replay as Marcus Rashford and Fellaini hit the net before James Tomkins halved the deficit. But, West Ham couldn't seal a comeback like they did against Arsenal in the Premier League over the weekend.
"I knew he [Martial] would miss because it was his left foot!" Fellaini told MUTV.
"I was stood in front of the goal and I just put my knee out towards the target. It was important in the end.
"It's always nice to score; it's good for confidence. Now we have a game on Saturday which we have to win in front of our home supporters."
Fellaini added: "When I was at Everton eight years ago, we played the semi-final against United and we won, so this time I hope I am on the winning side again.
"At a club like Manchester United, we have to win something. I think all the players want to do that this season."
Man United host Aston Villa in the Premier League on Saturday.