United boss Van Gaal has seen his defensive options severely limited by Tyler Blackett's suspension and injuries to Chris Smalling, Phil Jones and Jonny Evans.
Reserve-team captain Thorpe, 21, or 19-year-old McNair could therefore partner Marcos Rojo, who is likely to move inside from left-back, against Sam Allardyce's men.
Van Gaal confirmed the inclusion of the inexperienced duo before going on to explain why the versatile Daley Blind, a left-footer like Rojo, will not be utilised in the heart of the defence.
"They [McNair and Thorpe] are in the squad," Van Gaal told MUTV. "That is always why we have a youth education.
"It was also our policy [to utilise the youth squads] and always the policy of the club too.
"I don't like having too many left-sided defenders or left-footed players in defence.
"I think always of balance so I have to pick up a right-sided central defender from the youth education. Normally, I am always doing things like that. I have already said that we have a lot of vacancies when you buy only six players and let 14 players go.
"The amount of injuries is worrying me and they are not coming back. Smalling is already, for the third time, injured. Evans also. Jones had one injury in the national team, which was unfortunate.
"I am a coach who has a lot of experience and I believe I was never in this situation before so it is surprising me. I am evaluating myself and my staff and also the players and what is wrong.
"But I cannot imagine what we are doing is at another level to [what I did at] my former clubs. So I have already adapted to a certain level that I think we can play during a game but I cannot train at a lower level."
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