Aiming for a fourth straight home win in the Championship for the first time since 2007, the Reds had started well and had all of the chances in the first half until Swansea scored with their first genuine attempt in injury-time.
Ben Cabango looped in a header from a long throw-in but it was a glaring defensive mix-up between Mads Anderson and Michal Helik at the start of the second period that really cost them.
Swansea’s top scorer Jamal Lowe latched onto the loose ball after both defenders failed to clear and once he scored, it was always going to be tough for the hosts.
Ismael said: “It was a very strong first half, exactly to the game plan.
“We were in it, fighting very well and created a lot of situations.
“Just before half-time we conceded that goal and we had to chase.
“But we had to be more clinical as we created a lot of chances and a stupid second goal killed us.
“It was a mistake. It’s easy: just communicate, clear the ball and we’re still in the game. It’s frustrating. It killed the game.”
Replays suggested Lowe was offside and Ismael, who saw his side push leaders Norwich City close before losing 1-0 in their last Championship outing, said: “It was offside.
“And there was a penalty for us.
“But I don’t want to speak about this; it’s not about offside.
“We have to clear the ball and the failure is on our side and we have to deal with it and make it better as the next game comes quickly.
“We make them a gift. But it’s a good thing as we can compete at this level. We showed we can go 90 minutes against them. We just have to avoid such errors.”