They were utterly outclassed by their West Yorkshire derby rivals in an 11-try mauling and suffered a third straight Super League defeat.
Castleford were missing a raft of senior players and saw Greg Eden depart with a hamstring injury after 19 minutes but Powell offered no excuses.
They face Warrington Wolves next Saturday for a place at Wembley but he admitted: “The team out there is lacking any fluency or consistency.
“I’m not excusing anything; we were really poor defensively.
“Ultimately we have to get a team out there that defends better than tonight.
“Some of those defensive efforts are unbelievably unacceptable, and those boys won't be in the team for the semi-final, it is as simple as that.
"I've got to get a team out there who can do a job and get us to the cup final.
“I'm a proud man. I’ve always been a winner so it’s disappointing tonight and it is hard to put into words.
“It was tough to watch. But I remember in 2017 we beat them here by 60 odd points - and lost to them in the Grand Final.
“(Leeds coach) Brian McDermott mentioned that straight after the game back then.
“It’s not the end of the world. And we have opportunity next week to put that straight.
“It’s a great opportunity against a team bang in form.
“Clearly nobody will give us a chance next week and we have to turn it around pretty quickly.
“But there’s a couple of parallels which I think is a great reason why we have to remain calm.”
Powell brought scrum-half Danny Richardson off in the first half because of his defensive fragility as Leeds tore them apart down their right side.
It was Leeds’ biggest ever win at Wheldon Road but Powell said: “We’ve got three-quarters of our spine missing today.
“I've had to make a decision to take Danny Richardson off and put Macca (Paul McShane) at half.
“We conceded numerous tries there and I felt I needed to make a change there but he’s not on his own.”
Powell hopes to have Niall Evalds, Jake Trueman, Nathan Massey, Michael Shenton, Oli Holmes and Adam Milner back for the semi-final at Leigh.
He added: “For me, the players have to get their heads in the right place and trust each other that we can do the job.
“I know people are getting hot under the collar but they can’t be any more disappointed than me.
“We have to put that behind us and we’ve got three training sessions to get ready.
“Those players coming back, they make a big difference to the experience of the team.”