“We conceded a goal due to my players being careless,” Mansourian told reporters after the home loss.
“That is what I was afraid of during the whole 90 minutes of the match.”
Al-Ahli were good value for their 1-0 win which puts them in the driving seat for the return leg in Dubai on Sept. 16 but former Iran midfielder said his side, making their debut in the ACL, had not given up hope of making the last four.
“Al Ahli were smarter than us and managed to score a goal.
“We lost the first leg match, but we have some days to work on our weaknesses and instruct our players – especially the younger ones – on some factors to get everyone ready for the second match,” he said.
Naft, seeking to become the first Iranian winners of the tournament since PAS Tehran won the Asian Club Championships in 1993, did have their chances to level.
Brazilian defender Carlos Santos sent a header just over the bar two minutes after Lima’s strike, while skipper Alireza Ezati wasted another headed chance from Abbas Bozar’s free kick.
Ahli boss Cosmin Olaroiu said there was still plenty of work for his side to do in the second leg.
“There are still 90 minutes remaining and we not going to be easy,” the Romanian said.
“We know how strong Naft are with free kicks and at set pieces, so that we shouldn’t think that we are past this stage by now.”
Olaroiu’s former club Al-Hilal, runners-up last year, are likely opponents in the semi-finals for whoever advances after the Saudi side crushed Qatari champions Lekhwiya 4-1 in the first leg of the other West Asian quarter-final on Tuesday.
(Writing by Patrick Johnston in Singapore; Editing by Greg Stutchbury)