After fresh optimism at Old Trafford following Louis van Gaal's appointment as manager, United were brought back to earth on Saturday as Swansea City recorded a surprise 2-1 win in their Premier League opener.
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"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 so far this summer and, as exclusively revealed by Goal, have agreed a deal worth £16 million plus a loan for Nani to sign Sporting Lisbon's Marcos Rojo.