SCOTT Fitzgerald admitted Millwall's youngsters are struggling to make the step up to U21 level after watching the development side lose 4-0 at Cardiff City.
The Bluebirds side featured no less than ten players who had played for the senior side this season and Fitzgerald conceded his "naive" players were no match for the opposition's experience.
"Maybe the scoreline was harsh but they had a very strong side out," he said. "We had five or six second year scholars in our team, and they had none.
"It was a harsh lesson. We're making naive errors but that's because we've got young, naive players."
Striker Etien Velikonja, who Cardiff bought for in excess of £1million last summer, scored a perfect hat-trick, with his first coming on the stroke of half-time.
"That was a terrible time to concede," Fitzgerald said. "It was just like the first-team game against Birmingham.
"We conceded in the 45th minute and they came out for the second-half with the momentum. It knocked us back.
"We've found it tough this season because we're a very young group. I don't think we'll see until after Christmas whether the new league is benefitting us.
"The only way we can say that it's been a success is if we start getting players into the first team. I think it's going to be a long, hard season for us."