Louis van Gaal will choose between newcomers Tom Thorpe and Paddy McNair as he decides which centre-back will make his Manchester United debut against West Ham on Saturday.
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United are without three injured central defenders and the suspended Tyler Blackett, and manager Van Gaal has ruled out pairing the left-footed duo of Daley Blind and Marcos Rojo in the middle of the back four.
Thorpe, 21, made six appearances on loan at Birmingham last season and captains United's under-21 team while McNair is a 19-year-old Northern Irishman who can also operate as a midfielder.
"They are in the squad," manager Van Gaal told MUTV. "I don't [like] too many left-footed players in defence. I think always in [terms of] balance so I have to pick up a right central defender from the youth education."
Luke Shaw is set to make his belated United debut after being an unused substitute in the last two games as Argentina international Rojo, who had played on the flank, is set to move infield.
Van Gaal added: "Maybe last week he could have played already but it is better to give an injured player more time to recover. He is now ready to play and I am anxious to see him."
Centre-backs Chris Smalling, Jonny Evans and Phil Jones are all sidelined and Van Gaal said he is worried how injury-prone the trio are.
"Smalling is already injured for the third time," he explained. "Also Evans, I believe. Only Jones has one injury for the national team. I am a coach with a lot of experience and I was never in this situation. It is surprising me. I am evaluating what is wrong but I cannot imagine what we are doing at another level from my former clubs."
Record signing Angel Di Maria has scored in his last two games but Van Gaal said he is eager to see the Argentine do better at his defensive duties.
He said: "In ball possession, he is playing fantastic. He is creating a lot, he is scoring goals and he is giving assists so as a coach you cannot want more. But there is also a main moment in football where we lose the ball and there he can improve."