Victor Lindelof says Manchester United have struggled because of complacency this season.
United are currently seventh in the Premier League after an underwhelming start to the season.
Ole Gunnar Solskjaer's side appeared to have turned a corner in their most recent outing - a 3-1 win over Brighton which the manager described as the Red Devils' best performance of the season.
United are also the only team to have taken a point against Premier League leaders Liverpool this term, and have beaten second-placed Leicester City and third-placed Chelsea .
However, they have also suffered shock defeats to Crystal Palace , Newcastle , West Ham and most recently Bournemouth .
And centre-half Lindelof says they must treat every match as a big game if they are to rediscover the winning mentality that marked out the great United teams of the past.
"I generally think there is one thing we need to get better at if we are going to get back to the level Manchester United should be at: that’s winning matches week in and week out," the Swedish defender told Aftonbladet.
"Sometimes it does not have to be neat, but you should win anyway.
"There’s been a bit of a problem - If we go into the matches against big clubs, we’re all connected, and we get results.
"Then a few days go by and we meet a slightly lower team, then we have to go in exactly the same way, not get comfortable.
"We start well, play fast and get to positions – but then we stop with it after 20 minutes, complicate it and play poorly."
Lindelof will hope his team-mates heed his words when they return to action after the international break with a Premier League clash away to fifth-placed Sheffield United.