Sneijder missed two qualifiers in October last year through injury and then friendlies against Germany and Italy because of a lack of match fitness after being sidelined at Inter Milan over a contract dispute.
He joined Galatasaray in January but Van Gaal said Sneijder was not yet back to his best.
"Sneijder did not play for four months and is still improving but he can do much better than he showed so far," Van Gaal told reporters on Friday.
"Maher is in my opinion in the picture for the same spot as Sneijder and Van der Vaart," Van Gaal said.
"Now he will play in the under-21 team and we will see how he handles that."
Van Gaal handed a first taste of the senior squad to Feyenoord's teenage midfielder Tonny Trindade de Vilhena and included Jeremain Lens, despite the striker serving a domestic ban after a clash with fellow international Joris Mathijsen.
"I spoke with Lens about his behaviour just as I already did with Mathijsen.
"But a suspension from the disciplinary board of the KNVB does not have consequences for my squad as everyone has their sins.
"Besides that, Lens is the best option to replace the injured Klaas Jan Huntelaar as back up striker."
