In a clash which the Kiwis needed to either win, or at least perform well in, they dropped ball, missed tackles and gave away clumsy penalties as the Roosters ran riot at Mount Smart Stadium.
"I’ll admit that’s pretty disappointing, it’s probably one of the most disappointing games of the season really," McFadden said.
"We had everything to play for but we certainly didn’t come up with much today. It was well below our best and something we’ll definitely have to look into.
"I thought we lacked a bit of conviction with our play there. We had a plan that I think would have worked, but I didn’t see much of that so it’s something we’ll look into, but there were lots of other disappointing areas as well.
"I didn’t see it coming. I thought our preparation was really good but I think there is something there…I haven’t got any answers for you now."
The loss means the Warriors will almost certainly need to win both of their remaining two games, against the Titans at home and then the Panthers on the road.
Upset victories by the Eels and Cowboys this weekend added to the misery, and the New Zealanders now find themselves down in tenth position, two points behind eighth-placed Parramatta.
"It has to be [must-win from now on], the results last night…that’s going to make it tough for us, we are going to have to win both games and then maybe even require some other results to go our way," McFadden said.
"Certainly we have still got an opportunity and we are a better football team than that. We are going to have to find it this week.
"It is always a challenge the NRL, it is a long season and getting yourself up every week is a challenge, but we should have been up for that.
"That was a big occasion for us today and we needed to perform well against a very good side but we fell well short."
The New Zealanders ended the first half having completed 17 of their 18 sets and looked right in the contest before conceding five tries in the second period.
But McFadden said it was clear the Roosters wanted it more from the outset.
"I thought the signs were there in the first half to be honest. Pretty early in the game I thought we lacked energy," McFadden said.
"As a coaching group we need to take some responsibility for that. We will look at our programme to see if we over-cooked things or anything like that.
"There are no excuses really for that lacklustre performance."
The first-year coach also confirmed that winger David Fusitu’a’s season is likely over, after he dislocated his shoulder in the fourth minute of the match.
"It’s not looking [good] for Dave at the moment – he looks like he has dislocated his shoulder so probably season ending for him," he said.
"He has had a nerve impingement in his shoulder for a little while, but not a full dislocation like that."