In a see-sawing encounter the lead changed several times until a late try to Denny Solomona eventually sealed the Tigers’ 31-24 win.
Radford said Hull FC‘s sloppiness in contact “absolutely killed” his side.
“I’ve got no qualms with the result,” he told Sky.
“Our sloppiness after contact absolutely killed us tonigh. and I thought their backfield did a real job on us. That kid on the wing, Hitchcox, he dropped in last-minute and he’s looked an absolute world-beater out there today.
“Credit to them. Where we was in terms of contact, just not good enough.”
Winger Jy Hitchcox scored two tries on his Super League debut, a late addition to the side with Joe Monaghan pulling out of the warm-up with injury.
“That bar we’ve been at [in the first two rounds], we certainly weren’t there tonight,” Radford admitted.
“And it’s a shame because if we were we could have had a really good evening. But that’s not to be. Credit to Castleford, they’ve come to our home and schooled us in how to defend I suppose.”