Radhi Jaïdi conceded that tonight's abrupt Checkatrade Trophy exit will serve as a big 'wake up call' for his youngsters, who were comfortably beaten 3-0 by an experienced Southend side.
The goals all came within a 12 minute spell in the first half, with Jaïdi adding that naivety contributed to the gulf between to the two sides.
Tonight's victors, Southend, and Cambridge both progress from Group B, but the young Saints will have plenty of lessons to take from the tie, and a valuable taste of how demanding senior football can be.
"The only positive was not conceding a goal in the second half. It's a reality check for most of our boys," Jaïdi reflected.
"I know we want to develop players, but this is the level they have to compete at if you want a career in the game.
"I've played in the EFL and I know the demands at this level. We told the boys that it'll be tough, and they saw that tonight.
"From my understanding, we have good players who are beginning to mature. But, only in an academy environment. The next step is to go and compete on a senior pitch.
"You're playing against men, who are fighting for their careers.
"They had experience, no doubt. It wasn't an easy task for our boys, but we were hoping to see a reaction from the Cambridge game last month.
"Some players stood up to the challenge second half, but the majority need to learn more."