National football coach Bernd Stange was poised to praise his weakened side for a brave display.
For 80 minutes, the Lions held on stoically, preventing Syria from scoring the decisive second goal to kill off the Group A Asian Cup qualifier in Teheran on Friday.
Singapore even had a penalty claim turned down in the 80th minute.
Then, it all went to pieces for the visitors, who conceded three goals in the last 10 minutes.
"That's unacceptable," said Stange, speaking to The New Paper after the game.
"I was prepared to praise the team. At 1-0, I thought with some luck, we could get a draw with a counter-attack or set-piece.
"Instead, we gave up. For a German coach, it is a disaster to concede three goals in the last 10 minutes."
Singapore had beaten Syria 2-1 at the Jalan Besar Stadium last month to collect their first win in the group.
But missing eight players - including star central defensive duo Baihakki Khaizan and Safuwan Baharudin - through a combination of suspension, personal reasons, National Service commitments and disciplinary issues, Stange was forced to use even more under-23 players for the return encounter.
In all, nine players from the U-23s tasted action, with six in the starting 11.