The Red Devils shelled out around £150m in the transfer window in a summer that included the arrival of Angel di Maria from Real Madrid for a British record £59.7m.
The former Anfield hero admits that the Dutchman has plenty of options to choose from at Old Trafford, but thinks that it will be difficult to get the right combinations.
He told the Daily Mail: "If Van Gaal is going to make things tick for United, he is going to have to make some high-profile and tough decisions with his team selection.
"After an extraordinary end to the summer window, when they smashed the British transfer record with their £60million purchase of Angel Di Maria and then signed Radamel Falcao on loan, Van Gaal has a squad to start making progress."
United's new boys could make home debuts when they host Queens Park Rangers on Sunday.