The Frenchman scored the winning goal in injury-time as United defeated Everton 2-1 in their FA Cup semi-final clash at Wembley on Saturday.
Martial has scored a total of 15 goals for United in all competitions this season, but Van Gaal insists that he can still improve, and has denied suggestions that the Frenchman is a natural centre-forward.
After the victory, Van Gaal told reporters: "He (Martial) can do much better because I see still a lot of things he can improve on and he shall improve because only by training and playing can you improve.
"But also when you are playing at a higher level you improve also. When you are older you know all the things shall happen again. Then you have experience and can decide much better in the same situations as before.
"I don't think he is a striker, he is more a wing player. But maybe in the future [he could be a striker].
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"But I don't think so. He's another type of striker than Ruud van Nistelrooy or other examples of Manchester United strikers."
Van Gaal has come under pressure in recent months after a largely below-par season at Old Trafford, but says he is not prepared to discuss his future in public, despite holding talks with the club's board.
Asked about his future Van Gaal added: "I'm not looking beyond this competition.
"We live with pressure. For the future I have discussed that with my board and not with you."
Martial's coolly taken winner came after Chris Smalling's own-goal had cancelled out Marouane Fellaini's opener, with Romelu Lukaku seeing a penalty saved by United goalkeeper David de Gea prior to Everton's leveller.
United will face either Crystal Palace or Watford in the final, with the second semi-final taking place on Sunday.