Louis van Gaal says Manchester United are "suffering" from a hectic schedule and must improve on their second-half display against Wolfsburg.
United bounced back from a 2-1 defeat to PSV Eindhoven with a 2-1 win in their second Champions League Group B game on Wednesday night, a fourth straight victory in all competitions for the Premier League table-toppers.
It was not entirely convincing, however, with United having to come from behind after Daniel Caligiuri's fourth-minute strike and then, following Juan Mata's penalty and a Chris Smalling goal, hang on to their lead towards the end.
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"We have to keep our feet on the ground and we have to improve a lot to be at the top of Europe," Van Gaal said.
"We have beaten Wolfsburg, I am very happy and the players are very happy, but we have to improve on the second-half performance and we have to know that, and I have said that also to the players.
"That is why we are here -- to improve ourselves so we win every game.
"We could have won this game in a better way.
"However, I think we deserved to win because we created much more chances that Wolfsburg."
Van Gaal felt United got lucky when they were awarded the 34th-minute spot-kick for a handball by Caligiuri.
And the Dutchman suggested overall that United's team spirit had been a key factor in the win, while also claiming bad luck and tiredness had been to blame for some of their sloppy play.
"There are referees who are not whistling for such an incident so then you are lucky that he is whistling," Van Gaal said of the handball.
"But in the first half I was very pleased. We created so many chances -- but then you don't finish them, and that is unlucky.
"Then in the second half we continued with our game spirit and made a beautiful goal.
"But after that it was not good anymore, which is remarkable because keeping possession is something we have done very well for the last five or six matches.
"We had given a lot in the first half, and also, (it is about) the accumulation of the matches in the last weeks because I cannot understand (otherwise) why we would give the ball away so easily.
"But I think also the team spirit still kept us in the match."
United have had a midweek fixture for three consecutive weeks and face a trip to Arsenal in the Premier League this weekend.
"A lot of players are very tired because of the accumulation of a lot of matches," Van Gaal added on BT Sport.
"We score but after that it was suffering," he added.
"Arsenal played yesterday -- one day more to recover. That will benefit them but we shall manage it."