The Swans conceded a stoppage-time equaliser which denied them victory against Hull City at the weekend.
Sheehan has warned that the same lapses will be punished in their upcoming games against Nottingham Forest in the Carabao Cup in midweek and before they return to league action at Birmingham City.
The Swans twice took the lead through Zan Vipotnik and Ronald against Hull, but John Egan struck in the 97th minute to secure the visitors a 2-2 draw at the Swansea.com Stadium.
“It’s very disappointing, the manner of the equaliser. Seven [added] minutes was a long time to add on but ultimately we only played in patches,” said Sheehan.
“I will not hide from that. First half was very comfortable, but we were comfortable without doing a whole lot.
“We conceded more chances today than we have in the last four games – that’s not like us.
“Defensively, the longer the game went on we dropped, dropped, dropped.
“We tried to see out the game but we didn’t do that. We’ll take the point and there is a load to improve on.
“When you’re ahead after 96 minutes, no matter what happens you should see out the game.
“If we had done it would have been a wonderful, battling win. If not, so it’s obviously disappointing to concede like that.”
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The draw extended Swansea’s unbeaten run to six matches in all competitions and kept them seventh in the Championship table.
But Sheehan admitted that defensive lapses like the one against Hull cannot be repeated as his side prepare for a demanding schedule.
"We work on a lot of different things and I still think as a team we are simmering," he added.
"We were really good in patches today but I don't think we did enough to blow them away which is the disappointing thing.
"At times, especially how we started, I thought we looked really good but then it became a bit passive, a bit of a slower game and we didn't battle against that slow tempo."
Slovenian striker Vipotnik continued his purple patch with another goal – his fifth in six games for club and country – but Sheehan stressed that it is solidity at the other end of the pitch which will determine whether Swansea can sustain their early-season promise against some of the division’s strongest squads.
Hull boss Sergei Jakirovic, meanwhile, hailed his team’s resolve after ending a run of two straight defeats.
“We didn’t deserve to lose the game, especially after having so many chances in the second half. Their goalkeeper was unbelievable with his saves, just like in Bristol,” he said.
“What was bad for us was that we conceded a goal on the counterattack to allow them to take the lead 2-1.
“But I’m very happy for the lads because we didn’t surrender. We changed things with our substitutes.
“Between the 70-75th minutes we dropped a little bit and we couldn’t get the ball. So we changed and put more pressure on the ball and in the end we got our reward with a very important point.”
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