The winger has conceded his side’s performance during their fourth round clash at Old Trafford was disappointing, despite coming up against a full strength Red Devils side
A Chicharito double as well as goals from Ryan Giggs and Wayne Rooney proved too much for Martin Jol's side, despite Aaron Hughes' late consolation for the visitors.
Kacaniklic says that conceding early to a third minute penalty made it very difficult for Fulham to recover against a near-full-strength Manchester United squad.
"When you come to Old Trafford, the last thing you want to do is concede so early," Kacaniklic told the club's official website.
"It was the same last weekend at the Etihad Stadium when we faced Manchester City. At this level and against such high-quality opponents, it’s very difficult to recover.
"Okay, it's a tough place to play and United will always be the favourites at home, no matter who they play, but 4-1 was still a disappointing scoreline.
"My honest assessment is that we were just not good enough today. As a result, we’re now out of the cup – it’s a sad moment for us.
"We wanted to put a good run together, when you start out you always have faith that you can do well and go as far as possible. Yes, this was a tricky tie, but we still hoped to do better."
Fulham must now turn their attention to Premier League matters in the form of a midweek clash against West Ham at Craven Cottage, and Kacaniklic has admitted his side need to raise their game in order to bounce back from a string of poor results.
"January has been a strange month for us," he continued. "It started well with a win at West Bromwich Albion, and then of course, we went through against Blackpool in the previous round of the cup. There was the draw with Wigan Athletic also, but now we have lost our last two games.
"We know that we need to raise our game and a win against West Ham would put things back on track."