Louis van Gaal has warned Manchester United's rivals his side will only get better.
A 2-1 victory over Everton at Old Trafford on Sunday moved United into the top four for the first time since August last year.
David De Gea stopped Leighton Baines' penalty and made three late saves to deny the visitors a point, and van Gaal afterwards admitted the performance was patchy.
But the United manager sounded an ominous note when he said: "I've already told my players we aren't playing good and we've already hit the top four, so what will happen when we're playing well?
"I mean playing well for 90 minutes, because we've played well for half a game or 60 minutes.
"That needs time - I'm afraid to say it again, but you saw it. We need to improve the organisation because we were outstanding in the first half, but then we let them have so many chances.
"I made substitutions to help our organisation but despite my changes it was not better because in the last 15 minutes they created three big chances.
"David De Gea was fantastic today. When there are a lot of players in front of you your view is not so good as a goalkeeper.
"When the shot comes in from outside the box it is always at the last moment that you see the ball, but his reflexes are very good."
Radamel Falcao scored his first goal in English football to seal the win then spelled out his desire to remain at Manchester United beyond the end of the season.
The Colombian, who also cleared a Phil Jagielka off the line in the second half, is on loan from Monaco until the end of the season.
United will have to stump up £43million if they want to sign him permanently next summer and the Colombian hopes they do.
"Me and my family are happy here, and we would like to stay for many years at Manchester United," Falcao told MUTV.
(Sky Sports)