The Dutchman has given opportunities to several youngsters on the club's pre-season tour of the United States but concedes he is looking to replace the departed veterans
United have already recruited Luke Shaw and Ander Herrera this summer, while Van Gaal has given opportunities to youngsters Reece James, Michael Keane and Tyler Blackett on the club's pre-season tour.
But strengthening the defence remains a priority for United after the departures of experienced campaigners Nemanja Vidic, Rio Ferdinand and Patrice Evra this summer.
United are confident of completing the signing of Arsenal captain Thomas Vermaelen, but any deal has stalled due to Arsene Wenger's desire to confirm a replacement before sanctioning a departure.
Netherlands international Stefan de Vrij was heavily linked with a move to Old Trafford after an impressive showing at the World Cup in Brazil, but Lazio revealed on Tuesday they have completed a deal to sign him.
"I am looking at the defenders we have first," Van Gaal told reporters.
"And, when you see our three matches, we are doing well, I think. But it is right that we are looking for defenders, because a lot of defenders have gone.
"We have to replace them but I also like Michael Keane and Tyler Blackett."