James Tate's youngster was sent off favourite for the Molecomb at Goodwood but came off second best behind Karl Burke's speedy Havana Grey.
Invincible Army will now step back up to six furlongs, but Tate feels a sounder surface is of more importance.
"I don't think he liked the ground, but he went through it because he's game," said the Newmarket handler.
"That was why we dropped him back to five rather than run him over six in heavy ground.
"I was relatively pleased with him, but disappointed not to win all the same.
"He was on the wrong side of the track and had nothing to race with, so I think we'll go back up to six furlongs on nice ground hopefully.
"He's in everything, but in all likelihood he'll go for the Gimcrack next."