Former Manchester United first-team coach Rene Meulensteen has said that the club need "four top players" to return to their previous level.
Experienced boss Louis van Gaal brought optimism to the club this summer after replacing David Moyes at the helm.
However, Van Gaal's first competitive match in charge ended in disappointment, with Swansea City running out 2-1 winners at Old Trafford in their Premier League opener on Saturday.
"This is not going to be a quick fix," he told talkSPORT.
"They have still got two weeks to go in this transfer window. They definitely need to maximise the opportunities in this window and probably do something in January as well. This process will take a long, long time.
"We need to strengthen at the back, we need to strengthen in midfield and that is foremost the two areas they need to look at. In my opinion, they need to bring in at least four top players to crank up the level."
United have signed Luke Shaw and Ander Herrera this summer.
