Adelaide United coach Aurelio Vidmar believes the substitution of Alemao yesterday may have cost his team the game.

The Reds went down to Sydney FC 1-0 at the Sydney Football Stadium after Mark Bridge netted in the 77th minute but Vidmar is sure that if Alemao had stayed on then the result would have been different.

The Brazilian had to leave the pitch due to a stomach cramps, giving Vidmar no choice in the matter.

"He (Alemao) was complaining of an upset stomach and he didn't have any strength left," Vidmar said.

"I didn't want to take him off.

"It certainly changed the dynamics of the game.

"Even Michael Marrone was complaining at half-time about an upset tummy."

Vidmar rued the loss, saying it was a poor second-half performance that cost his team.

"I thought we were quite good in the first half and we created some good chances," he added.

"But in the second half we couldn't string a few passes together and that was a problem."

Adelaide United now has a win and a loss from their first two matches and sits sixth on the table.