Vidic was suspended for the games against Fulham and Villa
Defenders Nemanja Vidic and Jonny Evans have conceded Manchester United may have been complacent in their bid for an unprecedented quintuple of trophies.
United's Premier League lead has been cut from eight points to one and they drew 2-2 with Porto in the first leg of their Champions League quarter-final.
When asked if United had eased off in recent weeks, Vidic stated: "To be honest, maybe."
Evans said: "Maybe it's been our fault - maybe we've become a bit complacent."
When United beat Blackburn 2-1 on 21 February they moved eight points clear of second-placed Liverpool.
Manager Sir Alex Ferguson, whose side won the Club World Cup in December, attempted to cool talk of a quintuple after the Red Devils beat Tottenham to win the Carling Cup and since that triumph on 1 March they have suffered a 4-1 home defeat by Liverpool and a 2-0 loss at Fulham.
It required an injury-time winner from teenage striker Federico Macheda to overcome Aston Villa 3-2 at Old Trafford on Sunday and maintain a one-point advantage over Liverpool, although United have a game in hand.
If you look at the last four games then obviously we haven't played as we did in the 10 games before that - we definitely need to get back on the right road, we need to get back that mentality
Injuries and suspensions have hindered United, with Vidic banned for the matches against Fulham and Villa and Ferdinand ruled out of the games against Villa and Porto because of a groin problem.
Ferguson's men went 14 Premier League matches without conceding a goal before the victory over Blackburn but since then 10 goals have been let in during the past four games.
"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," added Vidic. "We didn't think we were relaxed but maybe we were.
"If you look at the last four games then obviously we haven't played as we did in the 10 games before that. We definitely need to get back on the right road, we need to get back that mentality.
"Maybe we need to work on our defence because in the last few games we have conceded too many goals.
"I don't think it's a case of players not having the confidence but maybe we've had so many games and also we've had players injured - at this stage of the season you need all your players to be ready to play the games.
"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."
Midfielder Michael Carrick feels United need to prove their mental strength from now until the end of the season but insists they have what it takes to overcome Porto at Estadio do Dragao, although the Portuguese side have never lost a home match against a British team in the Champions League.
"Of course we can progress," insisted 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."
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