Steve Bruce wants his Sunderland side to tighten up from set-pieces if they are to challenge for a top-half Premier League finish.
The Black Cats battled from a goal down to beat Blackburn last week, but they slumped to a 1-0 defeat at Stoke on Saturday.
Dave Kitson scored the only goal of the game just before half-time as Kenwyne Jones failed to sufficiently clear a corner.
Bruce admits his side's defending has been their Achilles heel, although he was pleased by their spirited second-half display.
"It's certainly been our downfall. We simply cannot defend the ball well enough from set-pieces," he admitted after the game.
"For a purist it was a poor game. We should be taking the nil-nil and going home.
"We had the best chances in the first half. In the second half we huffed and puffed, but the final ball wasn't up to scratch.
"It was a spirited effort in the second half, but we lacked that bit of quality.
"We've only been together a matter of weeks. Defending at set-pieces should be bread and butter stuff."
Bruce has not ruled out more signings before the transfer window stating: "We've got one or two things in the offing."