McDermott said he was “encouraged by the effort his team put in, but admitted a poor start – which saw Tigers go 12-0 up – proved costly.
It was three tries each and Rhinos held a two-point lead for most of the second half until Jake Webster’s touchdown won it for Tigers.
In a brief post-match press conference, McDermott said: “It’s disappointing, but I don’t have too many issues with the result.
“I thought we were off at the start, only for a short period at the beginning, but it got them field position and they scored a couple of tries which didn’t really go with the flow of the game, I don’t think.
“While they were good plays and Castleford deserved to score them, I didn’t think it was because they tired us out or got us on a play we couldn’t handle – we were just off.
“After that I thought we gave it a real go, I thought we gave that game our guts.
“I don’t have too much of an issue with the loss and I am encouraged there’s a group of men there that’s trying their hardest.
“They are absolutely trying and giving the game their guts, but the start was off for us.”
Captain Danny McGuire had been named in Rhinos’ initial 19-man squad, but did not make the 17.
He has not featured since suffering a knee injury in the opening game of the season, but has said he is hopeful of featuring at home to Wakefield Trinity Wildcats on Easter Monday.
Asked if McGuire will play in three days’ time, McDermott said: “Possibly, but I doubt it.”
Cas coach Daryl Powell said it was the “biggest” win of his time in charge.
Tigers were on a three-game losing run and missing a host of key players.
“That’s more like it,” said Powell, whose team had been hammered 56-12 at Warrington Wolves in their previous outing six days earlier.
“We didn’t get everything right, but we worked hard and the effort was magnificent.
“I asked the players for something special individually and they certainly delivered that.”