Steve Bruce says his team will be working on playing off the ball next week as he identified the major weakness against Leicester.
The Sunderland chief felt his side were sliced open far too easily by a Leicester side with no shortage of attacking spark.
The visitors won the game with goals from Fraizer Campbell and Jordan Henderson, who struck either side of Andy King's reply.
And while Bruce was delighted with his side's craft and flair going forward, he says work still needs to be done at the other end ahead of the big kick-off.
"I thought going forward we looked a threat all day and had that cutting edge with Bent and Campbell looking particularly sharp," he explained.
"But we must improve defensively, and I'm not just talking about the back four. The goalkeeper did terrifically well but he shouldn't have had to make so many saves.
"We've got to do better without the ball and that's what we'll be working on for the next two weeks because it's where we struggled today."
Campbell opened the scoring early on with his sixth goal of the pre-season, a smart finish across Leicester keeper Chris Weale.
But King powered through to equalise midway through the first half, only for Henderson to win it in the second half with a long-range blast which proved too hot for Weale to handle.
The Sunderland boss was impressed with Leicester, who made the Championship play-offs under Nigel Pearson last year and look a decent bet to do so again under new boss Paulo Sousa.
As for his own team, Bruce says they won't be able to get away with lapses in the Premier League when it kicks off again on August 14.
"You can see why Leicester nearly won promotion last year. They are an attack-minded team with some good players and I think they'll be there or thereabouts come the end of the season," he said.
"We sat far too deep, there were too many gaps and we were wide open. We know we can't play like that in the Premier League - we must be disciplined and organised.
"That will be a priority this week ahead of a big, big game against Hoffenheim."
