Nemanja Vidic has admitted that Manchester United were unhappy to again concede two goals in Sunday's game against Southampton.
The Red Devils needed to score three times to win for a second successive weekend as Robin van Persie completed a dramatic hat-trick at St Mary's.
Nigel Adkins' Saints had been 2-1 in front going into the closing stages and Vidic hopes the defence will tighten up quickly.
Many of the players will be away on international duty this week and the next chance for United to keep a clean sheet will come against Wigan on 15th September.
"It was a difficult game and we conceded two goals we're not happy with at all," Vidic told MUTV.
"We've been told before the game what Southampton's strengths are, what their game is and what they're trying to achieve. In the end, they did it so it's disappointing.
"I think it's only the second time in a year myself and Rio have played together so it's a long time and obviously it's tough as everyone needs more games.
"I have to say, we are not really pleased with the goals we've conceded. There was a lack of concentration, I think, for all four goals we lost [in the last two games] but we have to work on it.
"You have to say we've had a lot of turbulence in defence with so many changes and people going in and going out.
"We didn't have a constant back four and sometimes it causes a problem, but we look forward to the next games.
"We have over a week to improve our fitness and obviously our form so we are going to be right for the next game in the Premier League."
Vidic also paid tribute to Van Persie for bouncing back from a missed penalty to net a treble and seal United's victory.
"Robin was great," he added. "I think he's a great player. Even in his first game, he did a good job and this one is amazing with three goals to give us three points, which I think are going to count very much at the end of the season.
"He's a good penalty taker but it could happen to anyone. It was not a great penalty, I have to say, but, in the end, he managed to score two more so we got the win."