Jersey's 3-1 loss at Thamesmead Town was down to a lack of experience, according to coach Kevin McCarthy.
It was Jersey's second defeat by a non-league side in pre-season after their 3-0 loss to Sutton United last month.
"They had a bit too much experience for us, that's all that was in it," McCarthy told BBC Radio Jersey.
"Our lads did well but any time the ball broke in our half of the field we always looked under threat," added the former Jersey manager.
Tyrus Gordon-Young and Theo Fairweather-Johnson had given the hosts a 2-0 lead at half-time, before Rupert Murray volleyed Jersey back into it soon after the break.
But Gordon-Young grabbed his second goal to seal the win for Thamesmead, who play at the same level in non-league football as Guernsey FC.
MacCarthy, who was Jersey manager from 1999-2001, was in charge of the side as boss Jimmy Reilly was attending Goodwood races.
"It's a business commitment," Jersey FA president Phil Austin told BBC Radio Jersey of Reilly's absence.
"They're all doing this voluntarily and we understand that.
"Jimmy spoke to us and we've got other members of the management team that can step in so it's not a problem."