Marco Silva admitted he was at a loss as to why his Benfica team couldn't repeat their Hearts heroics after his side were forced to play behind closed doors.
Hearts welcome Benfica to Tynecastle on Thursday and require a footballing miracle after a disastrous 6-1 first-leg defeat in the Europa League last week in Lisbon.
But Benfica were surprisingly held to a 2-2 draw at an empty Estadio da Luz by newly promoted Academica de Viseu. The match was played with no fans after the hosts were punished for pyrotechnics last season.
And boss Silva admitted that caused them a problem, ahead of their visit to Edinburgh. He said: "The absence of fans had a big impact on the game, there's no hiding that, but there's no point in talking about it, it will sound like an excuse.
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"We have to look at what we could and should have done differently to win the game, even without fans. "But I also believe that scoring two goals at home should be enough to win.
"We had many opportunities, and with normal efficiency it would have been a game decided at half-time, but we didn't. There's no clear explanation why we were so effective against Hearts on Thursday but not here.
"Perhaps it was a lack of concentration because we didn't score the amount of goals we should have. But we should never have conceded two goals in the manner we did, and we have to work on that.
"We are frustrated and it's about channeling that in the right way in Europe this week now."
