Nemanja Vidic admitted Manchester United had been guilty of complacency and warned his teammates they must rediscover their focus if their season is not to end in failure.

In less than four weeks United have seen a seven-point lead at the top of the Premier League cut to just one and face a battle to reach the Champions League semi-finals after a 2-2 draw with Porto at Old Trafford.

While they top the league at home, remain in with a chance of making the last four in Europe and are one win away from the FA Cup Final, it would be churlish to talk of a crisis.

But, as Vidic conceded, these are worrying times for the club, whose 3-2 win over Aston Villa on Sunday - which restored them to the top of the table - served only to mask the underlying fragility in their squad.

Asked if United had eased off in recent weeks, Vidic said: "To be honest, maybe. Against Liverpool, maybe you can say we took it easier because we had a game in hand and there was a big gap in points.

"We didn't think we were relaxed but maybe we were. I don't like to keep saying we've had many games to play, because all teams do, but we have and I just hope our experience will see us through this." Injuries, suspensions, fatigue and a collective lack of confidence are the chief reasons for the recent wobble that is threatening to engulf United during the run-in and scupper their hopes of landing one of the big prizes.

On his first game back since his nightmare display in the 4-1 defeat to Liverpool, Vidic looked hesitant, as did the rest of United's defence, and Rio Ferdinand's recovery from a groin injury cannot come soon enough for them.

There is no doubt playing Porto 48 hours after the adrenaline-fuelled win over Villa, because of the demands of TV scheduling, had an adverse affect on United, even though manager Sir Alex Ferguson was able to make four changes.

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But not even that can explain the team's unconvincing record in Europe this season, which has seen the Champions League holders win just three of nine games, drawing the other six. Hardly the stuff of potential winners.

Add to that the fact no English team has ever won in Porto's Estadio do Dragao, and United's hopes of making it through next Wednesday's return leg look bleak, particularly as the Portuguese champions have two precious away goals.

"It's a big challenge to go to Porto and get through," said Vidic. "We were on course for a good result but we ruined that with a minute to go by conceding another goal.

"I don't think it's our confidence, but we started badly again against Porto. We've had many games this year - Wednesday then Saturday - and we've had a lot of players injured and some coming back from injury. We've definitely had a bad period, we haven't done well and we've conceded many goals.

"After the Liverpool game, that first bad game, we needed to fix it against Fulham but we didn't.

"But we need to think less about that now and concentrate on the next game. We don't need people to put pressure on this team, because we're still in three big competitions." United midfielder Michael Carrick admitted Fergie's men needed to prove their mental strength from now to the end of the season but insisted they have what it takes to overcome Porto.

"Of course we can progress," said Carrick. "We've got to win a football match, it's as simple as that. It's a tough competition and if you want to win this trophy you've got to overcome problems and obstacles along the way.

"We won't get too down about things, but put things right and be ready for the run-in. We're top of the Premier League, in the last eight of the Champions League and have an FA Cup semi-final coming up. That's certainly not a bad situation to be in."