Shamrock Rovers manager Stephen Bradley was left to rue the concession of four goals as the Hoops slumped to a 4-2 defeat to Flora Tallinn.
The Estonians will take their lead into next week's second leg, with the winner set to progress to the group stages of the Europa Conference League.
Graham Burke and Liam Scales scored the goals for the Dublin club in Tallinn.
However, Bradley was left frustrated at his side's defending.
"We gave away four terrible goals from our point of view and if you do that at any level, especially this level, you don't deserve to win the game and we obviously haven't tonight," he said post-match.
"We make really bad mistakes, mistakes I haven't seen this team before and obviously they have to capitalise on them and they did."
The Hoops boss said he and his players were left disappointed, despite some positives.
"We're not a team that concedes goals and especially not goals like that.
"If you come to a team and you get outplayed or cut open sometimes you have to accept it and take it on the chin, but tonight we gave away four really, really poor goals from our point of view so if you do that, you don't deserve to win so we're obviously disappointed.
"There were a lot of positives, but you still can't get away from the fact that we conceded four goals and that's not good enough, whether that's domestically or in Europe you don't concede goals like that so positives but still not good enough from us."
Still Bradley said he believes his side can turn things around in the second leg at Tallaght Stadium next week.
We're well capable of winning the tie, there's no doubt about that, but as I said it was individual mistakes tonight and it's very very rare for this team to give up goals like that, tonight we've given four up and that's not good enough.
"We've had this in Bratislava, it's half time we're two down, we know what we need to do to win the tie, I'm just disappointed tonight we've given away the goals we've given away."