The non-league side's eye-catching run in the competition came to an end on Saturday when they were outclassed by their Championship opponents at Kenilworth Road.
"I think we never gave ourselves a chance to put them under pressure," Buckle told ESPN.
"The early goal was a poor error by us, but we had a lot of chances in the game.
"There were a lot of openings in the 90 minutes for us, probably more so than we had in the last two [wins] against Wolves and Norwich so to concede early was a real cardinal sin."
Luton return to Blue Square Premier action on Tuesday night when they travel to Macclesfield Town.