Goals in quick succession to Sergio Cirio and Jeronimo Neumann inside the first half-hour laid a nice foundation, but the Reds were regularly pinned in their back third in the second half, conceding two late goals as the visitors had all the play.
Gombau pointed to Adelaide's short turnaround following their first round match against Perth Glory only five days earlier as a decisive factor.
"In the second half my team was very tired," he said.
"We were tired because we played on Sunday, (and had) just five days to recover.
"We played with nine (men) last game (against Glory towards the end).
"We also could not make a lot of changes because there are suspensions and injured players."
Although frittering away what appeared a match-winning lead, Gombau was pleased with the effort given the extenuating circumstances which, as well as the brief five-day break, included the late withdrawal of star playmaker Marcelo Carrusca (groin) and a foot injury sustained by right back Daniel Bowles after just 15 minutes of action.
"I am satisfied," Gombau said.
"In this position we lose a lot of players, we had no Cassio, no Carrusca, (Nigel) Boogaard and Isaias are under suspension and Bowles with his injury.
"But this is football.
"We are playing 11 against 11 and I don't any excuse.
"We are growing and learning from the mistakes we are doing.
"We are a team under construction."
Bowles' premature exit paved the way for talented teen Awer Mabil to play a more prominent role off the bench than Gombau had anticipated.
Pacy winger Mabil impressed with his energy and drive and smashed the left upright with one fierce strike, which fell into the lap of Jeronimo who ambled in for United's second goal.
The 18-year-old Kenyan, who attends St Columba College in the northern suburbs, was in a class of his own - in more ways than one.
"Mabil played an amazing game," Gombau said.
"He is very young. It's funny because this morning I called him to say he is in the group (team).
"But he didn't pick up the phone. After he called me back, I said 'hey, why didn't you take the phone?'.
"He said 'Josep, I am in school'. It's amazing that in the morning he is in school and in the afternoon he is playing here and he did very well."