Colchester United midfielder George Moncur sent a makeshift West Ham side to a narrow 1-0 defeat in their latest pre-season friendly.
With a UEFA Europa League qualfiying clash in Malta against Birkirkara on Thursday, the Premier League side named a squad made up entirely of youngsters to avoid any unwanted injuries to key first-team players.
A bright start from the visitors failed to produce a goal and after 13 minutes they found themselves behind as Upton Park youth product Moncur fired a free-kick into the top corner from 20 yards.
Colchester soaked up plenty of pressure after the restart before threatening themselves, Alex Gilbey denied by a fine save from Raphael Spiegel.
With time running out the young West Ham side began to push for a leveller and Marcus Browne went closest with two minutes remaining.
He was unable to find a way through, though, a problem Slaven Bilic will hope does not affect his senior side in Europe in two days' time.