Tony Carr believes defensive mistakes cost the Under-18s as their ten-match unbeaten run in the Barclays Under-18 Premier League came to an end on Saturday morning, when they lost 2-1 to Southampton at Staplewood.
The match started well for the Hammers who went ahead after 18 minutes when left-back Lewis Page finished off a flowing move, sending the ball into the far corner following Nathan Mavila's cut-back.
The visitors held a half-time lead but surrendered it on the hour mark when goalscorer Page fouled Tom Leggett and Marcus Barnes slotted home the resulting spot-kick.
Eleven minutes later and the Saints scored a winner when Josh Sims netted, leaving Carr ruing the side's second half display.
"Up until half-time we'd absolutely dominated the game and with the way the match was panning out, it was only a matter of time until we got the second goal.
"The second half didn't pan out that way, we missed a couple of good chances with a number of players missing opportunities.
"We had a bad five minutes where we conceded a penalty, which was sloppy defending from Page, and Amos Nasha got tangled up in a tackle where he should have done a lot better defensively and the boy goes through and scores.
"Really, defensively, you'd have to say they only had two shots on goal and scored two goals. It was a game we didn't deserve to lose, should have won but you end up getting punished for your inability to get that second goal."
Defeat bought an end to their unbeaten streak which stretched back to a 5-0 win over Liverpool on 28 September 2013.
Carr was disappointed to lose the run but believes the players could be rightly proud of their achievement.
"It was a disappointing way to lose that unbeaten run but nothing lasts forever, we have to move on and hopefully start another run.
"It's been an unbelievable run. We were going into every game believing we would win, playing with confidence, in the league we were as good as any team and it was just disappointing we weren't outplayed when we got beat. We just conceded two sloppy goals and lost."
The U18s' run propelled them up the league table to their current position of fifth, where they are now just five points off leaders Fulham and Carr says the team have cause to believe that they can mount a challenge at the top.
"If you're winning games then you're playing with confidence and that means that the players are playing well.
"They're at a stage in their careers where winning starts to become important . Hopefully we want some of the boys pushing on to the U21s as well so it's a challenging second half to the season for us all."
West Ham United U18s: Howes, Amoo, Page, Makasi, Onariase (Pike 71), Nasha (Tombides 77), Mavila, Cullen, Brown (Martins 71), Bywater, Parfitt-Williams.
Subs not used: Guzman, Oxford