Di Maria has scored just four goals in 28 appearances, and has been linked with a loan move to French giants Paris Saint-Germain following the £25m arrival of Memphis Depay.
Desailly told the Mirror: "Van Gaal will have made a list of three players in every position: first, second and third choices. It would then be for the management to go out and buy them.
"But [Jose] Mourinho already has all the first choices he wanted. It means he is covered for all of the tactical set-ups that he wants throughout the season whether it is in the Champions League or the Premier League.
"Van Gaal, maybe he has had second or third choices. Angel Di Maria may have broken the [British] transfer record, but was he Van Gaal's first choice? The opportunity was there, Real Madrid wanted to sell and United took advantage, but was he Van Gaal's first choice? We will never know.
"This summer, United know that they will have to try and find all the first choices for Van Gaal."