Chelsea's England midfielder Frank Lampard has warned his team-mates they cannot afford any more basic defensive errors when they face Manchester United on Sunday.
The Blues' indifferent form in recent weeks has seen them slip three points behind leaders Liverpool at the Premier League's summit.
Defeat against Manchester United at Old Trafford on Sunday could leave them six points adrift of the Reds who face struggling Stoke.
Chelsea have been working on defending from set-pieces in training this week as coach Luiz Felipe Scolari attempts to stamp out errors that are beginning to cost his team dearly.
''The errors have been very frustrating for everyone involved because we do work on them in training anyway,'' Lampard said.
''Sometimes it has been an individual error or a collective one, but it has been very important because we have dominated the last two games and we lost out on a set-piece in the final minute each time.
''We can't afford to let that happen against a team of Manchester United's calibre.''
Chelsea have conceded costly goals against Liverpool, Roma, Arsenal, Bordeaux and West Ham during the past three months.
Late goals in their last two games against Fulham and Southend have increased Scolari's concerns.
Chelsea squandered 15 chances to kill of plucky Southend in their FA Cup third-round tie last weekend and Scolari initially focused on their poor finishing.
However, the Brazilian coach has now turned his attentions to the opposite end of the pitch with more emphasis on defending free-kicks, throw-ins and corners.
Clint Dempsey was unmarked when he headed an unlikely last-gasp equaliser for Fulham and Ricardo Carvalho's poor header allowed Peter Clarke to earn Southend a third-round replay at Roots Hall next week.
Scolari has now added England Under-21 defender Michael Mancienne to his squad after a two-month loan spell with Wolves.
The Brazilian wants the youngster, who turned 21 on Thursday, to remain part of his first-team squad.
But Mancienne has not ruled out a further loan period in the build-up to the European Under-21 Championships this summer.
Mancienne is certain to be part of England's squad for the competition and he said: ''I need match fitness coming up to the Championships.
''But I am now where I wanted to be at 21 - around the Chelsea first team but also playing first-team football as well by going on loan and getting experience.
''I have the experience now of playing pressure football, football with lots of fans and the expectation to win games, so that has matured my game.''
Chelsea will welcome back captain John Terry for the trip to Old Trafford. The defender has missed the last three games through suspension after being sent off against Everton.
Midfielder Michael Ballack is also available again after serving a one-match ban against Southend in the FA Cup.