Portugal went ahead in the fourth minute through Ricardo Quaresma, and England equalised from Manchester City's Joey Barton.
Platt's team dominated after that but were undone by the hand of Helder.
Platt said his side's "backs were against the wall" after Friday's insipid 1-1 draw in Macedonia. The situation was not helped when Liverpool goalkeeper Chris Kirkland suffered a groin strain in the warm-up.
England's final game in Turkey next month is now meaningless and it could be a time for experimentation. Platt said: "I will go into the next game looking for a win regardless of the group.
"I have always said there are no friendlies at international level, but whether I go with a group of players that cannot play in the next campaign, I don't know. Whatever group of players do go, they will go there to win."